You'll need to see what switches are sending power to the score motor... Some easy things to check: Make sure all the score reels are resetting to zero. Check to make sure none of the coin chute switches are stuck closed... Next, check the switches that send power to the score motor. Check the following:

D relay switch - red/white wire and yellow/red wire
E relay switch - red/white wire and yellow/red wire
G relay switch - red/white wire and yellow/red wire
I relay switch - red/white wire and yellow/red wire
K relay switch - red/white wire and yellow/red wire
O relay switch - red/white wire and yellow/red wire
P relay switch - red/white wire and yellow/red wire
S relay switch - red/white wire and yellow/red wire
W relay switch - red/white wire and yellow/red wire
AX relay switch - red/white wire and yellow/red wire
Score motor switch 1C - red/white wire and yellow/red wire

Make sure these switches are clean and adjusted properly...

AX relays are annoying to adjust, if they are the least bit off, they give you trouble.

*****If this is your first EM, I'd browse http://www.pinrepair.com/em/index1.htm#top before I attempted much in the way of cleaning/adjusting. Sometimes you'll do as much harm as good.*****

That is probably a stuck playfield feature switch...like a drop target switch making contact or something.

I would start with pinrepair and review typical issues. Old EM's that have sat for a while get "gummed up" in spots from people putting grease or oil when they shouldn't and just from dust etc.

Usually the score reels need cleaning adjusting. Stepper units will need cleaning--they are famous for having a layer of crud on the rivets.

Before you clean any contacts, tighten the switch stacks. Then do your cleaning/adjusting.