On a workgroup network, I have one computer that I just can't figure out.

When you start up the computer, it'll log onto the workgroup just fine, and will stay connected for about 2 minutes. Then it'll terminate the connection with the workgroup ("network path not found"), and I can't see the other computers on the LAN after that; Other comps on the network can't see this comp either.

I need to do a cold boot to get this cycle going again. When the connection gets "dropped" while running Quickbooks, it gives me a "error code: 64" I'm baffled.

Checked the system time in case of sync problems, but that's not it.

Any ideas?

This could be a NIC issue, replace the current NIC with a known working NIC into the pc in question. You will have to play detective on this one, by breaking down possible causes down and investigating one at a time.