You mentioned the four magic words 'traffic jam' and 'no connectivity' – this to me sounds like a central switch problem of some sort. There is no reason why a traffic jam between one or two machine should bring down a whole network!
There is a good possibility that either one of your switches or hubs is doing the damage, or you've got a problematic network card that's simply spitting out garbage constantly and therefore brining the network to its knees.
I'd advise you to check the lead indicators on your equipment next time this happens to see if there is a particular device transmitting constantly and try disconnecting it to see if there is any effect.
Another thing you can do is download a copy of Ethereal, which is a free packet sniffer and see what's happening on the network when the problem occurs. You'll need to have some knowledge on packet sniffers if you're to examine the network packets, so if you haven't used one before, ask a friend to lend you a hand. If you like more information on switches, hubs and other devices that might be the cause of your problem, you can visit www.Firewall.cx where there is an extensive analysis on these technologies.
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