Some additional info:

We swapped out an entire engine unit (solenoids, hall sensor board, adjust board) and it did the same thing. He has brand new ribbon cables from the master to the slave, and a new PROM. We tried switching the mpu->master cable. No joy. Doubt it's the signals coming from the hall sensor, as the swapout should have taken care of that. I still can't quite figure out if the op-amps are for oscillation or not. To me, it looks like the u1b and c parts oscillate, but only the u3d does on the other side, which is...weird. I only have a loose grasp of this stuff, so I can buy that it's supposed to oscillate. But he's replaced all the chips, and it still does it. Which leads me to conclude it's either a bad vref from the mpu, or the mpu isn't strobing the enable like it (maybe) should. Because the only way BOTH sides don't oscillate is either those, or u1 failing. And he's replaced u1. Thoughts?

2) is correct.

We swapped out good parts into the non-working machine. Actually, we swapped out I'm-pretty-sure-it's-alright parts, as we've sold all the Corvettes we've owned so I couldn't test them independantly. I just grabbed them off the shelf. Marking every part good or bad is lightyears ahead of our current organisational level. But as there was absolutely no change in behavior during those swaps, I doubt there was anything wrong with them.

We did not. I don't have a spare slave one to swap in. He replaced the tip102s as well. He's also measured all the resistors. Which is why I lean towards mpu/master. Didn't swap out the mpu, as I didn't have a spare security one.

To rudely answer for him, it's a newly acquired machine. And the little board with the adjustment pot had a backwards plug on both engines, although I don't think that has anything to do with it.

Nah, I'm pretty sure the magnet input's fine. It makes total sense, looking at the schematic, for it to vary around the point of 2.5v. Um, somebody around here must have a corvette to trade out the master and slave boards. Because one of those has to be your problem. I mean, we've sold 3 of them in the past 5 years. They can't have all gone out of state.