I think this is server related because the errors happen on more than one workstation - no errors in the server event logs. Users intermittently (with one user it happens every afternoon almost like clockwork) get disconnected from the server/ad and usually restart and reconnect just fine. They get 40960 and 40961 errors in the event logs. I've tried various things in terms of registry settings, network card driver, etc., and can't seem to make them stop. Unfortunately, this happens intermittently so my troubleshooting usually involves trying some changes and then coming back in a day or two to check logs and see if it happened again (which, so far, it always has).
Another thing - and this might be a clue: On the problem workstations it seems that the DNS server changes. The server is serving DHCP and DNS, and the only DNS entry according to the DHCP server is itself (192.168.1.2). However, sometimes I will check a workstation with ipconfig /all and find that DNS has changed to two servers - the router (192.168.1.1) and one other external server. Yet the DHCP is still showing as the server (192.168.1.2). I don't frankly know how the DNS server can change on the fly. I can do an ipconfig /release and ipconfig /renew and it comes back to 192.168.1.2 like it should be. For my latest attempt, I hardcoded the DNS on a couple of computers to 192.168.1.2. I'll check tomorrow and see if they've changed again!
Thanks in advance for any ideas.