First thing I do is make sure all stepper units work correctly.
Clean all biscuit rivets/fingers/clean lubrication... etc.
All relays have tightened switch stacks.. etc.
Exercise Jones plugs...etc.
I usually unplug the playfield and manually advance the units to simulate game play.
Check relays for missing brass washers... stuck relays could happened.
Check schematics to find all associated contact/relays that activate the gameover.