I'm just hoping someone has run across this before, maybe my search terms were bad but I haven't found anything quite like this issue.

The symptom: During play, the sound will quickly fade out over ~2 seconds and at the same time all solenoids lose power. After a variable but usually short time, say 20-45 seconds, the sound and solenoid power returns with the current game still in progress. It generally happens after a couple minutes of play. Lamps and score displays remain unaffected. He hasn't had it happen when on the lower PF, but he doesn't spend much time down there so I can't rule it out. The only other oddity reported is that the shoot again light remains on for the entire game after being lit, it awards the extra ball and just stays on until another game is started. Probably not related, but odd considering the new boards.

Work done on his game:
Ground mods to all backbox boards
All electrolytic caps replaced top and bottom
Aftermarket MPU, Driver, and Power board (I can't remember the brands ATM)
Extra grounds added on the connector between the MPU and Driver
Pull up resistors added to lower PF transistors
Edge connections on old boards cleaned with eraser (no crimps replaced)
New bridge rectifiers
All LED except chase lamps
All grounds tied together before the ground plate in the cabinet
That's all I can think of at the moment.

I'm really hoping someone has seen this behavior before and can point me in the right direction.

Thanks!

I think I did put new crimps on those connectors but I'll double check next time I'm there.

I had tested the aux power board and was missing some of the voltages during the problem, although I can't remember exactly which as it was some weeks ago.
As best I can tell, the main power to the aux board comes from the 24vDC bridge, the 12vDC bridge, and the 12vAC from the transformer. So no real connection to the 38vDC power there. Although I see now that the 24vDC powers the T, Q, and U relays. So something is likely pulling down the 24vDC line?

I can't confirm that the backglass lamp is also on, but yes, it stays on into the game over status.

So at this point my idea is to inspect this circuit on the aux power board, along with the connections leading to it.gdfgs (resized).png

I'm all ears for more thoughts though.

I think I did put new crimps on the power board connectors as that was the only new board I installed. So new board, I accidentally said re-pinned above, I'll edit the post.

I really appreciate all the responses! I'll set up a time with my friend to dig in further soon.

So the problem with the "soft reset" was the rottendog power board replacement, it has no 5v adjustment and was perfectly at 5v allowing almost no room for sag. Something was occasionally dragging the 5v down just enough to cause things to fade out, but not enough to reset the game. I rebuilt the original power board, set it to 5.1v, and all is well. Silly me for thinking a new board would be more reliable.