What could possibly be the cause?

Ok, I have a strange situation that keeps popping up. For some reason, unknown to me, we will have computers start giving an error message saying that "user cannot log in, domain unavailable." I have found that logging in to the local machine at that point and then unjoining and rejoining the domain fixes the issue. Any thoughts on what could be causing this issue and what could possibly be a more permanent fix?

 

Did you check the event logs for hints?