currently no logon servers available

Hello,
We have a number of Win 2000 pro computers who from time to time loose all connections to network resources and get the error message “there are currently no logon servers available”. This happens after users have logged onto the domain for no apparent reason. We solve the problem by getting users to log off and log back on again but can anyone tell me what causes this problem in the first place. We use static ip addresses in a small lan enviorment. Thanks.