User computer disconnect from domain

I am facing this problem some time when user open his/her computer early in the morning its showing same computer is not connected with domain, i can access only local admin, its not happen oftenly only some time, so all i do to go to local admin and put computer on workgroup and restart then again connect with domain this way its work but again give problem after few days, i am using windows 2000 for users and for domain we are using windows 2003 enterprises kindly help me in this subject thanks