Kicked off Network

This is a very strange problem, which Microsoft won’t reconize as “a problem”. I am running Windows 2000 on two servers, for no reason at all, at various times of the day, myself or a client will suddenly not be able to access maped drives. It tells them access denied, almost as if they had been logged off the network? The only way to solve this problem that I have found is to reboot the PC!?! I have heard of other companies experencing this problem and returning to NT, has anyone else heard or experenced this, and if so, your advise is very welcomed!!