Does anyone have a different IM (like Yahoo?), and if so, does it experience the same problem?
If something was sucking up all of your network bandwidth for just long enough to disconnect the IM clients and no longer, it would be very difficult to use IE at the same time to see if that was also down; by the time you launched the browser or went to your home page, it would be back up.
From a software perspective, I would give a hard look at all of the client machines (if that's possible) to see if maybe one of them had a virus or spyware that was periodically spewing a bunch of garbage out to the internet and using all up your bandwidth. For instance, as people leave and shut off their machines, does the problem go away? You can easily check how many packets a machine is sending out and receiving, if one of them is totally out of whack, that may be the one.
Hardware-wise, if you have a cheap or old firewall, I've seen them go nuts and do stuff like that (flicker out for a second). Low end routers and hubs are also likely suspects.
Hope this helps!
"I would rather have a free bottle in front of me, than a pre-frontal lobotomy..."
-Shrubble
