SYMPTOM
Sometimes I lose power or get partial power to flippers and/or the two maze kickers -- I don't think I've seen the pops, drops, or the slings lose power even once. It's happened off and on pretty much since I got the game. I can go 10 games with no issues, then the problem comes back. I can turn it off a while, then on again (didn't count the time interval to imagine heat changes) and then it might work fine again for a few games.
OTHER WEIRDNESS
kinda loud hum in attract mode *before* you start a game after first power up.
WHAT I'VE DONE
* reflowed SDU-100 header pins
* added 3 grounding mods to back of SDU-100
* verified solenoid fuse and under-playfield fuse have continuity out of socket (replaced playfield fuse for good measure)
* tried jiggling all 5 SDU female connectors while flipping flipper that is dead (no change) -- but only tried this once
* Disassembled both maze kickers and cleaned em.
* swapped in another SDU-100 to verify that the problem in fact goes away (which at least eliminates female connectors + other boards and fuse clips)
WHAT I HAVE NOT DONE
* measured TPs while in a good or bad state
REFERENCE
https://www.pinwiki.com/wiki/index.php?title=Bally/Stern
http://techniek.flipperwinkel.nl/ballyss/rep/index1.htm
https://pinside.com/pinball/forum/topic/vids-guide-quick-bally-driver-board-repair-bulletproofing/
READING
https://pinside.com/pinball/forum/topic/stern-pinball-troubleshooting
https://pinside.com/pinball/forum/topic/flippers-dying-mid-game
https://pinside.com/pinball/forum/topic/strikes-and-spares-solenoid-power
https://pinside.com/pinball/forum/topic/bally%E2%80%99s-hotdoggin-solenoid-issues
https://pinside.com/pinball/forum/topic/how-to-test-flipper-relay-on-solenoid-board-ballystern
https://pinside.com/pinball/forum/topic/classic-bally-flippers-not-working-stumped
https://pinside.com/pinball/forum/topic/bally-eight-ball-outhole-kicker-not-working
https://pinside.com/pinball/forum/topic/stern-meteor-flippers