I have two ADSL lines from completely different providers (Demon and The Phone Coop (www.thephone.coop), both behaving the same. The lines are both in the Peterborough area but on different exchanges. Although this isn't a Linux problem I think Linux is most likely to give me the tools to resolve it.
Basically, some of the time the ADSL lines just work: I can browse the web, collect email, download ISOs, etc.
However the rest of the time (and its becoming increasingly "most of the time") I can't. To all intents and purposes the connection has gone. Except, and this is what's throwing me, connections which were already up will continue to work (eg if I have an SSH connection somewhere it'll still work, I use Hamachi (www.hamachi.cc) as a VPN client and I can still access my home PC from work and vice-versa through it when the connection is otherwise useless, if I'm downloading an ISo using BitTorrent it'll usually continue just fine). The ADSL router isn't showing anything unusual (I have different hardware at home from at work and have swapped both with no effect).
All I have managed to do is: (a) Confirm that DNS is not the problem. DNS doesn't work properly during an "outage" but equally I can't access sites/email/etc by IP address either (b) Small packet stuff seems OK. Often (not always) I can ping a site which I can't view. (c) Following on from the above, if I tweak the ping packet size so that fragmentation occurs the ping becomes unreliable. (d) Optimised sites like Google are far more likely to work than non-optimised sites with lots of data.
There's no obvious patterns to the time of day/day of week (non-peak times are probably better but its not clear cut). Rain makes things worse (but our office ADSL line has never liked rain, and I think that's a different problem).
Any idea what is going on or how to find out? The ISPs so far haven't been much help, but since I have the same problem with two different ISPs I don't think its an ISP problem. A traceroute from my office to somewhere and from my home to the same place don't show any obvious points of commonality until they reach the destination (ie I'm pretty sure that apart from going through BT cables the two ISPs aren't sharing the same hardware/connectivity somewhere).
At home my router uses an embedded Linux which I can SSH into, so suggestions for things I can try from there welcome. It uses BusyBox and has limited tools but there may be some useful stuff under /proc if I knew what to look for.
PS: This has been going on for a few weeks but is definitely getting worse.