unexpected account lockouts

I have a strange situation with Windows 98 Clients. We currently are on a Windows NT domain, and we have several clients that run Windows 98. These clients map a drive to a personal folder on a Windows 2000 Server. We have noticed lately that the clients will sporadically be denied access to their personal drive, while still logged on to the network. They receive an “access denied” message. If the clients then log out, and log back on, their account is locked out. Anyone have any ideas why?