virtualbox.org

Are they set to use UTC in the guest settings? Mine are not.

Could you post a log file for one of the NT machines (as an attachment)
The one I see here is the w2k3 so if it is holding leave it alone for a bit.

I don't know what happened. But the day after my last post, the machines have caught up and stayed in synch. Even the NT boxes are matched up within half a second to the host machine, and I haven't seen any deviation for days.

I used to tell all of my students. If it ain't broke, don't try to fix it.
Consider yourself lucky.