A little follow-up: few issues since my last post but the other day I had a crash, and another failure to boot (no flashing lights this time). Because I already ordered replacement 68B09 CPU's I just went ahead and installed those. There was no memory corruption afterwards.
This leads me to suspect that the ribbon cable from the voice board to the pot board was not to blame after all...
Maybe it was a CPU issue all along? Who knows. Posting here to help others who might encounter the problem.
Cheers, Jacob