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.