OP

Quoted from mikoz:

Hi all,
I measured the thermistor's resistance at "cold" (well, fairly cold... it has been a few hours since it was used) and it's 4.7 ohms... this is higher than the 2-3 ohms expected and would explain things I think. A high value means it's limiting the voltage to the xformer more and I'm still betting that a low voltage will cause flaky behavior. Mine must not be too bad, because I think if it's really bad it can cause game resets.
Next I will turn it on and check the AC voltage across it, but I am thinking this is a formality. The voltage across it should be < 1VAC during normal operation. I am guessing I will see higher. A higher voltage drop implies more limiting is being down to the xformer, which would cause the rectifiers to see less power which would cause the entire array of circuits to see less voltage... it may just be low enough that only certain things are affected.
I should also mention that when I start up at cold, I don't always see all three failures, sometimes i only see one:
1. row 2 gnd short
2. row 3 gnd short
3. idol lock (but I think this is because the optos are on one of the affected rows)
Furthering my theory is that it's summer here in Texas, and that means lower line voltages to start with due to the unofficial "brownout" that results from all the ACs going hard. We typically dip down to 110V in the summer (which itself is probably fine, but not with a bad thermistor) and stay at over 118V in the winter. So, in the winter, at least for me, the chances of seeing this are probably smaller, I am guessing.
I suppose I can jumper around it temporarily to be sure at the risk of pushing the rectifiers a bit, but I ordered some of these from mouser (unfortunately Frys doesn't sell anything like this):
https://www.mouser.com/ProductDetail/Amphenol-Advanced-Sensors/CL-30?qs=sGAEpiMZZMuqZeNK75brD%2frjMwDFP8g4kbJYciAX3hY%3d

OP

Quoted from mikoz:

Well, I just jumpered the thermistor and the problem remains. It may have been a bad thermistor, but that's not the only problem unfortunately.
I so wanted that to be the issue, and it made perfect sense.
I am now at a loss without a scope.

OP

Quoted from mikoz:

Hi
So it seems I have two problems: thermistor...but this isn’t the real problem, and a bad 10 opto board.
I went from a stable machine to all these issues all within a month: mpu board (corrosion damage), poa opto board, thermistor, 10-opto board.
Crazy stuff
I ordered a new 10-opto board and will update when it’s installed. For now, I bypassed the thermistor until the new ones arrive.
The good news is that I now have spare a 8-driver board, 3-opto board, power driver board and poa motor board.

OP

Quoted from mikoz:

Hi all
10 opto board installed. 4 boots with no issues! Also installed new thermistor, but that’s not the root cause of the problem.
Thanks!!