I haven't been back to work on it but here are the notes of what he (the owner) did so far:
It's working with two flippers, full game and no resets. Right now my suspicion is with the thermistor based on this from the doc:
Resetting while the game warms up is therefore a key indicator of a failing thermistor.
I went through what testing I felt comfortable doing (will document that in a bit) and the last step I did was 4.8.11, checking the thermistor. It looked good but after reading about it I decided to try letting the game warm up. Right away I started a game and made an effort to use only one flipper at a time. Soon enough I got into the game that I figured I had probably used both so I tested and sure enough there was no reset. I played several games, got mist to work a few times, coffin multiball, castle lock (sort of) and most everything else worked. Tomorrow I'll start it up and try both flippers right away to see if it resets before warming up. I'm hoping so since a thermistor failure sounds like it would be the nicest point of failure.
Right now the only credit dot error is Switch 44 - Trough 4 Balls. Makes sense since it still seems to lose track. When a ball is in the castle lock it doesn't know what to do and has to search.
Everything I checked looked good according to the doc:
Line voltage: 122
Checked screws on PDB
Replaced the four 3A SB fuses. The old ones were each good but had some corrosion (though the fuse holder looked fine) that could have interfered but a test after that wasn't any better.
Reseated all connections on PDB. Didn't see anything obviously loose/corroded. One weird thing I noticed is that J116 and J118 have the same colored wires and same key so they'd be easy to get swapped. Since they are simply providing 12v and ground maybe it doesn't matter. J116 is for the coin door and J118 is for the 10 Sw. Opto board.
4.8.9 TP2 was 4.99 volts and the ROM's 32 pin was 4.97.
4.8.10 Rectifier was between 8.3 and 9 and the cap was between .505 and .562
I didn't pull the PDB since I don't trust I could recognize bad solder points anyway.
After all of this I still saw resets and then I unplugged and checked the thermistor but didn't make any changes.
Well, unfortunately BSD worked without trouble immediately after starting this morning, meaning I can't necessarily blame the thermistor. I then swapped outlets in the power strip (since I had unplugged it at one point so maybe it had moved) but couldn't find one to cause resets. On the first start there did seem to be a delay in the DMD displaying where I thought it was resetting but didn't really seem to. I'll keep watching for resets/oddness.