Welcome aboard the EM forum.
I would read this before starting any repair work as you will get a better understanding of how they operate and their inner workings.

http://www.pinrepair.com/em/index.htm

There are 2 stepper units on the bottom board in the main cabinet. One is the ball count unit that keeps track of the number of balls played, next to it is the coin unit that tracks the number of players. In it's home position, the coin unit is a on a one player game. In the back box with the score reels there is a player stepper unit. This works with the coin unit routing the power to the correct score reels based on the number of players. Read over the details in the pinrepair.com suggested by pin-it. Play close attention to the stepper unit section. Based on your description, I believe the player unit in the head of the game needs cleaning.

Grüezi Klaus
Photos von "Prospector" kann ich leider nicht beibringen. Er ist ein Williams-Klon, gute (?) Beschreibung hier: http://www.flippermarkt.de/community/forum/showthread.php?t=161542

Not pleasing Your word "sometimes" --- this usually means "a wire has broken-off somewhere - wire stays in place (connection, sometimes), but due to shaking the pin / vibration the wire moves a a bit - NO connection".
BUT - Your problem is "Player-Unit not only gets one shot of electricity" - sometimes the Unit gets TWO shots. And this means: The Score-Motor does make a turn too much. (Unfortunately) Your problem is strange. How often is "sometimes" ?
It would be helpful if it is "often" - so You can look into the pin - do tests and (often fault happens) see the faulty stuff. Greetings Rolf