К сожалению ничем больше Вам помочь не могу.
Мы все свои тесты проводим либо на poptop,mpd и pptp серверах наших устройств.

собственно, у меня и есть poptop:
[root@mega ~]# pptpd --version
PoPToP v1.1.4-b4
[root@mega ~]# pppd --version
pppd version 2.4.3
[root@mega ~]# uname -a
Linux mega 2.4.33-rc1-anp8-mega-p4 #2 Сбт Июн 17 03:16:12 NOVST 2006 i686 unknown unknown GNU/Linux

как дальше будем решать проблему?

P.S. собственно, мой сервер сдесь абсолютно непричем.
если ваш роутер поднимает соединение, и разрывает его через секунду сам, то ошибка в его логике работы,
и уж это никак не связано с каким-либо pptp-сервером - т.к. фаза установки pptp соединения успешна,
и даже успешно устанавливается ppp-соединение.
т.е. копать нужно как минимум в сторону pppd, но никак уж не pptpd

Будем мерятся конфигами и логами.
Завтра предоставлю лог со своего poptop'a и лог подключения.

Лог соединения poptop
----------------
Sep 19 13:19:44 baron pptpd[28924]: GRE: accepting packet #1103
Sep 19 13:19:53 baron pptpd[28924]: CTRL: EOF or bad error reading ctrl packet length.
Sep 19 13:19:53 baron pptpd[28924]: CTRL: couldn't read packet header (exit)
Sep 19 13:19:53 baron pptpd[28924]: CTRL: CTRL read failed
Sep 19 13:19:53 baron pptpd[28924]: CTRL: Reaping child PPP[28925]
Sep 19 13:19:53 baron pppd[28925]: Modem hangup
Sep 19 13:19:53 baron pppd[28925]: Connect time 15.3 minutes.
Sep 19 13:19:53 baron pppd[28925]: Sent 76320 bytes, received 76388 bytes.
Sep 19 13:19:53 baron pppd[28925]: Connection terminated.
Sep 19 13:19:54 baron kernel: device ppp0 left promiscuous mode
Sep 19 13:19:54 baron kernel: audit(1158657594.009:44): dev=ppp0 prom=0 old_prom=256 auid=4294967295
Sep 19 13:19:54 baron pppd[28925]: Exit.
Sep 19 13:19:54 baron pptpd[28924]: CTRL: Client 192.168.100.226 control connection finished
Sep 19 13:19:54 baron pptpd[29155]: CTRL: Client 192.168.100.226 control connection started
Sep 19 13:19:54 baron pptpd[29155]: CTRL: Starting call (launching pppd, opening GRE)
Sep 19 13:19:54 baron pppd[29156]: pppd options in effect:
Sep 19 13:19:54 baron pppd[29156]: debug # (from /etc/ppp/pptpd-options)
Sep 19 13:19:54 baron pppd[29156]: domain msk.dlink.ru # (from /etc/ppp/pptpd-options)
Sep 19 13:19:54 baron pppd[29156]: dump # (from /etc/ppp/pptpd-options)
Sep 19 13:19:54 baron pppd[29156]: auth # (from /etc/ppp/pptpd-options)
Sep 19 13:19:54 baron pppd[29156]: refuse-eap # (from /etc/ppp/pptpd-options)
Sep 19 13:19:54 baron pppd[29156]: name pptpd # (from /etc/ppp/pptpd-options)
Sep 19 13:19:54 baron pppd[29156]: 115200 # (from command line)
Sep 19 13:19:54 baron pppd[29156]: lock # (from /etc/ppp/pptpd-options)
Sep 19 13:19:54 baron pppd[29156]: crtscts # (from /etc/ppp/options)
Sep 19 13:19:54 baron pppd[29156]: local # (from command line)
Sep 19 13:19:54 baron pppd[29156]: asyncmap 0 # (from /etc/ppp/pptpd-options)
Sep 19 13:19:54 baron pppd[29156]: mru 1470 # (from /etc/ppp/pptpd-options)
Sep 19 13:19:54 baron pppd[29156]: mtu 1470 # (from /etc/ppp/pptpd-options)
Sep 19 13:19:54 baron pppd[29156]: lcp-echo-failure 30 # (from /etc/ppp/pptpd-options)
Sep 19 13:19:54 baron pppd[29156]: lcp-echo-interval 5 # (from /etc/ppp/pptpd-options)
Sep 19 13:19:54 baron pppd[29156]: hide-password # (from /etc/ppp/options)
Sep 19 13:19:54 baron pppd[29156]: ipcp-accept-local # (from /etc/ppp/pptpd-options)
Sep 19 13:19:54 baron pppd[29156]: ipcp-accept-remote # (from /etc/ppp/pptpd-options)
Sep 19 13:19:54 baron pppd[29156]: ipparam 192.168.100.226 # (from command line)
Sep 19 13:19:54 baron pppd[29156]: ms-dns xxx # [don't know how to print value] # (from /etc/ppp/pptpd-options)
Sep 19 13:19:54 baron pppd[29156]: nodefaultroute # (from /etc/ppp/pptpd-options)
Sep 19 13:19:54 baron pppd[29156]: proxyarp # (from /etc/ppp/pptpd-options)
Sep 19 13:19:54 baron pppd[29156]: 172.50.50.234:172.50.50.50 # (from command line)
Sep 19 13:19:54 baron pppd[29156]: nobsdcomp # (from /etc/ppp/pptpd-options)
Sep 19 13:19:54 baron pppd[29156]: noipx # (from /etc/ppp/options)
Sep 19 13:19:54 baron pppd[29156]: pppd 2.4.4 started by root, uid 0
Sep 19 13:19:54 baron pppd[29156]: using channel 24
Sep 19 13:19:54 baron pppd[29156]: Using interface ppp0
Sep 19 13:19:54 baron pppd[29156]: Connect: ppp0 <--> /dev/pts/16
Sep 19 13:19:54 baron pppd[29156]: sent [LCP ConfReq id=0x1 <mru 1470> <asyncmap 0x0> <auth eap> <magic 0x9eb38925> <pcomp> <accomp>]
Sep 19 13:19:55 baron pptpd[29155]: GRE: accepting packet #1
Sep 19 13:19:55 baron pppd[29156]: rcvd [LCP ConfReq id=0x1 <mru 1460> <magic 0x4100000>]
Sep 19 13:19:55 baron pppd[29156]: sent [LCP ConfAck id=0x1 <mru 1460> <magic 0x4100000>]
Sep 19 13:19:57 baron pppd[29156]: sent [LCP ConfReq id=0x1 <mru 1470> <asyncmap 0x0> <auth eap> <magic 0x9eb38925> <pcomp> <accomp>]
Sep 19 13:19:57 baron pptpd[29155]: GRE: accepting packet #2
Sep 19 13:19:57 baron pppd[29156]: rcvd [LCP ConfRej id=0x1 <asyncmap 0x0> <auth eap> <pcomp>]
Sep 19 13:19:57 baron pppd[29156]: sent [LCP ConfReq id=0x2 <mru 1470> <auth chap MD5> <magic 0x9eb38925> <accomp>]
Sep 19 13:19:57 baron pptpd[29155]: GRE: accepting packet #3
Sep 19 13:19:57 baron pppd[29156]: rcvd [LCP ConfAck id=0x2 <mru 1470> <auth chap MD5> <magic 0x9eb38925> <accomp>]
Sep 19 13:19:57 baron pppd[29156]: sent [LCP EchoReq id=0x0 magic=0x9eb38925]
Sep 19 13:19:57 baron pppd[29156]: sent [CHAP Challenge id=0xac <190998984d50de5a0d974b23a262e3737e4e7cb675daba>, name = "pptpd"]
Sep 19 13:19:57 baron pptpd[29155]: GRE: accepting packet #4
Sep 19 13:19:57 baron pptpd[29155]: GRE: accepting packet #5
Sep 19 13:19:57 baron pppd[29156]: rcvd [LCP EchoRep id=0x0 magic=0x4100000]
Sep 19 13:19:57 baron pppd[29156]: rcvd [CHAP Response id=0xac <0556394020223c8a47b819aca06919b1>, name = "test"]
Sep 19 13:19:57 baron pppd[29156]: sent [CHAP Success id=0xac "Access granted"]
Sep 19 13:19:57 baron pppd[29156]: sent [CCP ConfReq id=0x1 <deflate 15> <deflate(old#) 15>]
Sep 19 13:19:57 baron pppd[29156]: sent [IPCP ConfReq id=0x1 <compress VJ 0f 01> <addr 172.50.50.234>]
Sep 19 13:19:57 baron pptpd[29155]: GRE: accepting packet #6
Sep 19 13:19:57 baron pptpd[29155]: GRE: accepting packet #7
Sep 19 13:19:57 baron pptpd[29155]: GRE: accepting packet #8
Sep 19 13:19:57 baron pppd[29156]: rcvd [LCP ProtRej id=0x3 80 fd 01 01 00 0c 1a 04 78 00 18 04 78 00]
Sep 19 13:19:57 baron pppd[29156]: Protocol-Reject for 'Compression Control Protocol' (0x80fd) received
Sep 19 13:19:57 baron pppd[29156]: rcvd [IPCP ConfReq id=0x2 <addr 0.0.0.0> <ms-dns1 1.2.3.4> <ms-dns3 0.0.0.0>]
Sep 19 13:19:57 baron pppd[29156]: sent [IPCP ConfNak id=0x2 <addr 172.50.50.50> <ms-dns1 192.168.100.1> <ms-dns3 192.168.100.1>]
Sep 19 13:19:57 baron pppd[29156]: rcvd [IPCP ConfRej id=0x1 <compress VJ 0f 01>]
Sep 19 13:19:57 baron pppd[29156]: sent [IPCP ConfReq id=0x2 <addr 172.50.50.234>]
Sep 19 13:19:57 baron pptpd[29155]: GRE: accepting packet #9
Sep 19 13:19:57 baron pptpd[29155]: GRE: accepting packet #10
Sep 19 13:19:57 baron pppd[29156]: rcvd [IPCP ConfReq id=0x4 <addr 172.50.50.50> <ms-dns1 192.168.100.1> <ms-dns3 192.168.100.1>]
Sep 19 13:19:57 baron pppd[29156]: sent [IPCP ConfAck id=0x4 <addr 172.50.50.50> <ms-dns1 192.168.100.1> <ms-dns3 192.168.100.1>]
Sep 19 13:19:57 baron pppd[29156]: rcvd [IPCP ConfAck id=0x2 <addr 172.50.50.234>]
Sep 19 13:19:57 baron pppd[29156]: Cannot determine ethernet address for proxy ARP
Sep 19 13:19:57 baron pppd[29156]: local IP address 172.50.50.234
Sep 19 13:19:57 baron pppd[29156]: remote IP address 172.50.50.50
Sep 19 13:19:57 baron pppd[29156]: Script /etc/ppp/ip-up started (pid 29160)
Sep 19 13:19:58 baron pppd[29156]: Script /etc/ppp/ip-up finished (pid 29160), status = 0x0
Sep 19 13:19:58 baron kernel: device ppp0 entered promiscuous mode
Sep 19 13:19:58 baron kernel: audit(1158657598.270:45): dev=ppp0 prom=256 old_prom=0 auid=4294967295
Sep 19 13:19:58 baron fprobe[29172]: [INFO]: interface: ppp0, datalink: LINUX_SLL (113)
----------------
uname -a
Linux baron 2.6.17 #4 SMP Tue Sep 5 17:54:47 MSD 2006 i686 GNU/Linux

На самом деле странный параметр mru, попробуйте его увеличить до 1470

попробовал mru 1400, 1470
потом mtu,mru 1400,1470

никаких отличий не заметил.