I haven't had any issues like that with my miners on Middlecoin. Also, IIRC they don't switch coins that often.
Since you've overclocked everything, the first thing I would do is set it all back to defaults and see how cgminer behaves. If it's stable, then try slowly increasing the settings.
Good luck
Yeah, I've been thinking about that. What bothers me is that the last night's freeze happened at the same time (as far as I can tell) on both boxes. Could be a coincidence, need more data in order to be certain. I'll try lowering the GPU clock at least on my Win7 rig before I go to sleep, and leave the BAMT box running as a control. Thanks for your answer!