I turned off Logon Optimization. We have roaming profiles, so it should be off in the domain anyway. I have also explicitly mapped the drives persistent. So far that hasn't solved the problem. I doubt that the NAS is the problem, as it is brand new. It could be network, but we have been revamping the network lately, so we have been paying particular attention to network performance. That's partly why this is so frustrating: the infrastructure is pretty close to optimum. Even AD seems to be working well. To have this kind of random glitch is inexplicable.