OK - Do both target banks drop all the yellow targets, leaving just the black target raised ? Sounds to me like the game thinks it is in the "bonus" mode - if that is the case. So the game is simply "confused" as one of the switches on the score motor is out.

I'm with Millsy - the score motor contacts need to be cleaned.

Peach - This is covered in clay's guide ;

http://www.pinrepair.com/em/index2.htm#motor

You just need to clean the contacts - not adjust.