If some of the issues persist after disconnecting the MPU's J3 cabinet switch connector as suggested by Grant above,
then indeed look for a potential miswiring of the playfield switch matrix.
Or, switch matrix short to GI.
See my older post on another Stern here:https://www.maaca.org/viewtopic.php?f=22&t=18271&p=155916
1- Ensure the Credit/start switch is wired OK (and not physically/mechanically or electrically to the nearby SLAM switch).
2- Also check for any switch/diode possibly shorting to ground inside the cash door.
3- Go to switch test mode, and with no switch activated (all drop targets up, no ball in the saucers or outhole),
manually activate the playfield switches one at a time and record their numbers. Compare to the expected switch numbers in the manual.
Any funky unexpected numbers for a row or column? This should help you pinpoint the trouble area.
Looking for Bally Star-Jet, Capersville or The Wiggler, Williams Magic City, Grand Lizard, Swords of Fury, 1986 Pinstar Gamatron, and a few others.
Collecting & repairing electro-mechanical and solid state pinball machines with quality and care for over 35 years.
Serious and fun hobby, powerful nostalgia !