Ok, more work today:

o I replaced the three chips relating to the BEVNT signal (two quad opamps and a multivibrator) and the clock signal is now stable but only about 42Hz, not 60. The 7270 is set to ignore the signal anyway.

o System still exhibits the same random-character problem.

I'm hoping to get a spare M8043 (DLV11J) and M7270 from a friend of mine to aid in the troubleshooting. At this point I feel that swapping the boards is the only way to narrow it down.