disconnecting from network

What causes Windows 2000 Pro to disconnect and reconnect? I have gradually been trying to upgrade from Winnt 4 to Windows 2000 Pro and every system that is now Windows 2000 disconnects and reconnects. The funny thing is, even though they have a big fat red X on their mapping you can still access the drive and the X goes a way for a while. Everything I have read says put Service Pack 3 on. I have done that and it doesn’t help this issue one bit.

The other thought I had was a time sync issue, I still have Winnt 4 for my PDC and BDC. What problems can this cause?

Please help!!!!!!!!!!!!!!!