When I Had this problem on my WPC-S system it was a blown LM339 chip on the opto board.

When I bought a new one I bought a handful, because they aren't that expensive, and I never wanna go through the shitshow that was finding the problem. Sooo annoying.

See below. If it were me, I would order some online and then go aboot the fixing you're going to be doing anyway, and if you need them, you have them. It could be a ton of things...but whenever I hear "drop targets firing repeatedly" it make me think opto issues.

-What Goes Wrong?
The biggest problem for these boards is switch matrix shorts. If any of the power lines hits the switch matrix, it can easily destroy the LM339 and possibly its output diode. This issue will manifest as a ground short on one or more rows. It can be tested by removing the switch matrix connector. If the problem goes away, the associated LM339 and possibly its diode need to be replaced. -

http://www.pinwiki.com/wiki/index.php?title=Williams_WPC#Miscellaneous_WPC_Boards

EDIT: That's if you're sure the 12v power is fine on the Power board

Hmm...got anyone near you with a similar machine you could hook your opto board up to and test?

Unless someone can tell us how to test a LM339 chip that's installed, you're gonna have to get to someone close...ANYONE?
Such a bitch of a problem this one, I hated it.

Typical.

Those manuals suck ass...they're always lying in them!