Did you change any rubbers or clean the playfield?
Check to make sure no switches are close to touching and the vibration is setting things off.
Did you also change any bulbs under the playfield?
Go back under and make sure nothings touching.
Tons of stuff to short to the GI on medusa.
Also the 0.5 mfd capacitors on the pop switches and other quick action switches (if they are still there, sometimes cut off)
If there are any capacitors on the switches try cutting off one leg. The machine will work fine without them. But you can replace them when you get a chance if this is the problem.
When the caps go bad wierd things happen. Could be the problem if its not closed switch vibration because of new rubbers.
Bad switch caps wont show up in solenoid test. Only while playing.