I have a THOMSON speedtouch 576 router and recently I suffered from this.

Problem causing this was the Antivirus module of NIS2007.

My i.s.p. said to me there was not disconnection.

They said it did not show up upon the network as being disconnected.

So I would notice that it would only happen when I had windows live messenger or I was checking via Thunderbird (emails), it would show up as processing a threat.

This only started happening when I installed the latest build of NIS2007 in January 2007.

So I investigated further and tested my system without any AV or a Firewall or both and it was o.k.

So I noticed it happened when AV was processing a threat.

I gave symantec email help grief and then pointed out to them the exact problem.

I have noticed in past 2 weeks since they uploaded for subscribers to download some files fixing the firewall and since then no problems atall.

I will get to the point if you haven't already got it, I think 99% of the problems are software related.

Usually the hardware is o.k.

However correct the i.s.p. could have a problem and or the exchange, so focus on the software first and if it's not that then Badger the i.s.p. to sort out and run line tests.