Перешел на новый тариф Стрим (Мейнстрим, 4 мб). Все бы хорошо, но есть проблема: раз в 15-20 минут соединение разрывается и затем устанавливается заново. С прежним, 3-х мегабитным тарифом, такой проблемы не было.
В техподдержке Стрим сказали, что судя по тому, что они видят, модем сам разрывает соединения.

Вот лог-файл модема:
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Jan 5 23:11:44> Connection terminated.
Jan 5 23:11:44> ppp0
Jan 5 23:11:44> pppd 2.4.3 started by root, uid 0
Jan 5 23:11:49> Connect: ppp0 {--} nas0
Jan 5 23:11:49> peer from calling number 00:30:88:00:3D:49 authorized
Jan 5 23:11:49> ppp0
Jan 5 23:11:49> WAN IP address 85.140.65.182
Jan 5 23:11:49> PPPoE Connect with IP Address 85.140.65.182
Jan 5 23:11:49> PPPoE Connection Successfully Established
Jan 5 23:11:49> PPPoE Connect with Gateway IP Address: 85.140.64.1
Jan 5 23:11:49> WAN gateway 85.140.64.1
Jan 5 23:11:49> primary DNS address 212.188.4.10
Jan 5 23:11:49> secondary DNS address 195.34.32.116
Jan 5 23:38:47> DSL out of sync
Jan 5 23:38:52> DSL Carrier is down
Jan 5 23:39:12> DSL in Sync
Jan 5 23:39:12> DSL Carrier is up
Jan 5 23:39:14> PPPD Terminated Through Signal
Jan 5 23:39:14> Connection terminated.
Jan 5 23:39:14> ppp0
Jan 5 23:39:14> pppd 2.4.3 started by root, uid 0
Jan 5 23:39:19> Connect: ppp0 {--} nas0
Jan 5 23:39:20> peer from calling number 00:30:88:00:3D:49 authorized
Jan 5 23:39:20> ppp0
Jan 5 23:39:20> WAN IP address 85.141.166.154
Jan 5 23:39:20> PPPoE Connect with IP Address 85.141.166.154
Jan 5 23:39:20> PPPoE Connection Successfully Established
Jan 5 23:39:20> PPPoE Connect with Gateway IP Address: 85.141.164.1
Jan 5 23:39:20> WAN gateway 85.141.164.1
Jan 5 23:39:20> primary DNS address 212.188.4.10
Jan 5 23:39:20> secondary DNS address 195.34.32.116
Jan 6 00:06:23> DSL out of sync
Jan 6 00:06:32> DSL Carrier is down
Jan 6 00:06:48> DSL in Sync
Jan 6 00:06:49> DSL out of sync
Jan 6 00:06:52> DSL Carrier is up
Jan 6 00:06:53> ppp0
Jan 6 00:06:58> Terminated: Modem Hang-Up
Jan 6 00:06:58> Connection terminated.
Jan 6 00:06:58> Modem hangup
Jan 6 00:06:58> pppd 2.4.3 started by root, uid 0
Jan 6 00:07:02> DSL Carrier is down
Jan 6 00:07:18> DSL in Sync
Jan 6 00:07:22> DSL Carrier is up
Jan 6 00:07:22> pppd 2.4.3 started by root, uid 0
Jan 6 00:07:28> Connect: ppp0 {--} nas0
Jan 6 00:07:28> peer from calling number 00:30:88:00:3D:49 authorized
Jan 6 00:07:28> PPPoE Connect with IP Address 85.141.165.199
Jan 6 00:07:28> PPPoE Connection Successfully Established
Jan 6 00:07:28> PPPoE Connect with Gateway IP Address: 85.141.164.1
Jan 6 00:07:28> WAN IP address 85.141.165.199
Jan 6 00:07:28> WAN gateway 85.141.164.1
Jan 6 00:07:28> primary DNS address 212.188.4.10
Jan 6 00:07:28> secondary DNS address 195.34.32.116
Jan 6 00:07:28> ppp0
Jan 6 00:34:29> DSL out of sync
Jan 6 00:34:42> DSL Carrier is down
Jan 6 00:34:54> DSL in Sync
Jan 6 00:35:02> DSL Carrier is up
Jan 6 00:35:03> PPPD Terminated Through Signal
Jan 6 00:35:04> Connection terminated.
Jan 6 00:35:04> ppp0
Jan 6 00:35:04> pppd 2.4.3 started by root, uid 0
Jan 6 00:35:09> Connect: ppp0 {--} nas0
Jan 6 00:35:10> peer from calling number 00:30:88:00:3D:49 authorized
Jan 6 00:35:10> ppp0
Jan 6 00:35:10> WAN IP address 85.141.166.113
Jan 6 00:35:10> PPPoE Connect with IP Address 85.141.166.113
Jan 6 00:35:10> PPPoE Connection Successfully Established
Jan 6 00:35:10> PPPoE Connect with Gateway IP Address: 85.141.164.1
Jan 6 00:35:10> WAN gateway 85.141.164.1
Jan 6 00:35:10> primary DNS address 212.188.4.10
Jan 6 00:35:10> secondary DNS address 195.34.32.116
Jan 6 01:02:05> DSL out of sync
Jan 6 01:02:12> DSL Carrier is down
Jan 6 01:02:31> DSL in Sync
Jan 6 01:02:32> DSL Carrier is up
Jan 6 01:02:33> PPPD Terminated Through Signal
Jan 6 01:02:33> Connection terminated.
Jan 6 01:02:33> ppp0
Jan 6 01:02:33> pppd 2.4.3 started by root, uid 0
Jan 6 01:02:39> Connect: ppp0 {--} nas0
Jan 6 01:02:39> peer from calling number 00:30:88:00:3D:49 authorized
Jan 6 01:02:39> ppp0
Jan 6 01:02:39> WAN IP address 85.141.164.27
Jan 6 01:02:39> PPPoE Connect with IP Address 85.141.164.27
Jan 6 01:02:39> PPPoE Connection Successfully Established
Jan 6 01:02:39> PPPoE Connect with Gateway IP Address: 85.141.164.1
Jan 6 01:02:39> WAN gateway 85.141.164.1
Jan 6 01:02:39> primary DNS address 212.188.4.10
Jan 6 01:02:39> secondary DNS address 195.34.32.116
Jan 6 01:29:42> DSL out of sync
Jan 6 01:29:52> DSL Carrier is down
Jan 6 01:30:07> DSL in Sync
Jan 6 01:30:12> DSL Carrier is up
Jan 6 01:30:13> PPPD Terminated Through Signal
Jan 6 01:30:13> Connection terminated.
Jan 6 01:30:13> ppp0
Jan 6 01:30:13> pppd 2.4.3 started by root, uid 0
Jan 6 01:30:19> Connect: ppp0 {--} nas0
Jan 6 01:30:19> peer from calling number 00:30:88:00:3D:49 authorized
Jan 6 01:30:19> PPPoE Connect with IP Address 85.141.167.249
Jan 6 01:30:19> PPPoE Connection Successfully Established
Jan 6 01:30:19> PPPoE Connect with Gateway IP Address: 85.141.164.1
Jan 6 01:30:19> WAN IP address 85.141.167.249
Jan 6 01:30:19> WAN gateway 85.141.164.1
Jan 6 01:30:19> primary DNS address 212.188.4.10
Jan 6 01:30:19> secondary DNS address 195.34.32.116
Jan 6 01:30:19> ppp0
Jan 6 01:57:18> DSL out of sync
Jan 6 01:57:22> DSL Carrier is training
Jan 6 01:57:32> DSL Carrier is down
Jan 6 01:57:43> DSL in Sync
Jan 6 01:57:52> DSL Carrier is up
Jan 6 01:57:53> PPPD Terminated Through Signal
Jan 6 01:57:54> Connection terminated.
Jan 6 01:57:54> ppp0
Jan 6 01:57:54> pppd 2.4.3 started by root, uid 0
Jan 6 01:57:59> Connect: ppp0 {--} nas0
Jan 6 01:57:59> peer from calling number 00:30:88:00:3D:49 authorized
Jan 6 01:57:59> ppp0
Jan 6 01:57:59> WAN IP address 85.141.165.196
Jan 6 01:57:59> PPPoE Connect with IP Address 85.141.165.196
Jan 6 01:57:59> PPPoE Connection Successfully Established
Jan 6 01:57:59> PPPoE Connect with Gateway IP Address: 85.141.164.1
Jan 6 01:57:59> WAN gateway 85.141.164.1
Jan 6 01:57:59> primary DNS address 212.188.4.10
Jan 6 01:57:59> secondary DNS address 195.34.32.116

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Модем куплен в октябре 2006 года, прошивку не менял.

Судя по логу, "не тянет" линия. Частые обрывы самого ADSL соединения -> PPP тоже переподключается.

А есть способ это выяснить точно?
И второй момент: обрывы идут с интервалом около получаса (если точно - то 27 минут), что подозрительно периодично.

Попробуйте обновить прошиву до последней (20070322) на ftp.dlink.ru

Спасибо за рекомендацию. Если ситуация не изменится, то я так и сделаю. Кстати, а были случаи, что сам модем не держал скорость, а новая прошивка помогала?

Вспоминая светлое детство и модем (dial-up) курьер v.everything, товарищ Дягилев специально для него выпускал две версии прошивок - для хороших линий и для удовлетворительных/плохих. Видимо, не зря он так делал.