I have inherited a client running SBS 2003 R2 and a mix of WinXP and Win7 clients. All machines have the latest updates.

I started getting complaints that 1 or more computers would 'drop off' the network and be unable to access the server (network drives or exchange) during this time.

When this happens, I can't ping the client pc from the server.

A number of things fix this:
 - Rebooting the server
 - Rebooting the switch and router
 - Rebooting the client
But it always returns in a few hours.

We've tried re-cabling, it didn't help. We've replaced the switch and router and patch cables. I've disabled all TCP offload/chimney etc, updated the server's network card drivers and turned off power management on the network card.

What does work, is if I start Ping -T individually to each of the affected clients, the network will run indefinitely without problems. As soon as I stop the ping, I start getting complaints a few hours later.

Any ideas?