Workstations disconnecting from server

This just started to happen about a week ago. We have 6 workstations running Win 7 on Dell OptiPlex 908 and a Dell Power Server running win 2008 server R2 Standard. The workstations are disconnecting from the server at various times. Sometimes you can click refresh and its reconnected but other time you have to reboot. Checked the cable with cable tester and all seems fine there. Check the NIC cards with numerous tests and they seem fine. I checked the server but there is nothing in the logs about clients disconnecting. Nothing that I see changed in a week. Really not sure how to proceed to correct this problem.