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.

Since the sound board shares the encoded solenoid signals, unplug the sound board and see if symptoms changes.

Otherwise put the game in solenoid test, take note of any solenoid firing out of order. Sounds like you have encoded solenoid data issue(typically an open line).

Does it do it as soon as a ball kicks into the shooter lane when you press start.
The ball trough solenoid is fireing and causing either vibration triggering switches. Or the caps. Or a lampsocket is touching something it shouldnt.

Start with simpelest to check and work up to the more difficult things to troubleshoot.

Check.all.your lamp sockets etc. Make sure nothing is touching. And make sure nothing is touching any of the switches. There is so much going on under medusa that its possible a lamp socket is causing a short to a switch, a coil. Or any of the wires might be touching something. Etc, etc.

Check the simple stuff first. And check those caps on the switches or cut them all off. One bad switch capacitor will.cause havok to early bally games,

Medusa, motordome, fireball classic, escape from the lost world. Etc. All have this problem, and the caps cause the symptoms you are describing.

The caps are leaking so they show good in switch test. But in a game they momentarly short and cause coils and switches to register.
Cut one side off of all of them, check the coindoor, slamtilt, tilt too. The game will still work. Then replace them if it fixes the problem.

switch2.jpg

Did you ever find the culprit , because my medusa has the same problem .