Not sure if you're familiar with the utils so here's a couple of good pages.
DCDiag Tutorial Examples. Check your Windows Active Directory (+ Free Download)
NetDiag Tutorial. Check Server Network Connections. Free Download Support tool
Do the servers use Intel Network Cards? Did you update the drivers before it started?
Apart from something common (like sophos) an idea did occur to me.
You don't have any of the event logs set to manual clear do you? BAD things happen when an event log fills up.
speckytecky (25th September 2008)
It will really come down to even logs but I'd pay special attention to DNS and AD replication service message.
We had a similar issue a while ago because one of your DCs which is also the print server was running out of non-paged RAM due to a memory leak in a HP printer driver. When the system got low on non-paged RAM the DNS service was the first to fall over which in turn took down logins and eventually the whole network.
Got an external support contracter looking at the problem today, he thinks it's DNS but I will update the thread when we know more.
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