I vaguely recall having the same issue on my King Pin years ago. From memory, there is a bonus unit under the playfield around where the target assembly is. Tighten any switch stacks and clean the contacts with a business card.
It was a problem that took me ages to find - but the "unit" was right in front of the target assembly.
It is giving you a random 5000 problem because when the target is lit and dropped it scores 5000. All other targets score 500.
I wish I still had my King Pin !
