Motor control code running on my bare Uno (nothing but USB connected) has been running continuously since my last post. Serial seems stable, so I need to reframe the problem and start a new thread. Looks like it might've been suffering some kind of brown-out with all the motors hooked up, but I'm not sure why as all my supplies are isolated. Could I be getting ripples on the 5v line, and why would it take hours to manifest?
[RESOLVED - insofar as the comms failure is now known to not be a software issue]
EDIT...
It WAS a serial/USB issue in the laptop/OS itself. I tried another laptop (also XP SP3) and the problem VANISHED!