Mmmm... so long as I use a totally independent power supply for the motor side of the house, it works flawlessly now. Well, except for once in a very blue moon, when I force the motor in reverse while the current tries to drive it the other way, but even then it takes a few dozen attempts to glitch it.

That's on our MythTV box, which has an undervoltage PSU (4.8V on the "5.0V" line). On my notebook (which has 5.1V on the 5.0V line) I cannot glitch it in that way.

It's good enough now, but I really don't want another wall wart in that room. Powering the motor side directly from the 5V line of the ATX PSU in the box can still glitch it. So I may try a 12V line instead, and use a 7805 to step that down to 5V on the motor board.

The saga continues..