Have you given the Jones plugs a good cleaning?

Here, for more info on reels: http://www.pinrepair.com/em/index2.htm#reels

I'll need to see a high quality scan of the schematic from (for example) Staples ($2) or Kinkos ($6)

When the Score Reels are only working intermittently are the 1, 10 and 100 Point relays working reliably? In other words do the relays always fire when targets are hit or do they only fire intermittently? That would help narrow down where the issue is.

While a schematic will help as @HowardR mentioned there may be clues in other schematics like this one from Cabaret for example:
Cabaret score reels (resized).jpg
Since the Score Reels reset reliably we know that the Reset circuit at the top is working. If the 1 Point relay is working reliably I'd carefully inspect the Player Unit in the red box which is a step unit that determines which player will get the points being scored currently. Unless the Player Unit is clean and stepping smoothly it could make the Score Reels appear to be intermittent.

/Mark

Hi Vincent
to work on a pin I like Williams pins the most - mechanical and electrical reasons as well as the MANUAL. Look in ipdb https://www.ipdb.org/files/2408/Williams_1968_Student_Prince_Manual.pdf - scroll down to page-21 (ori-19) --- see the huge relay bank - and the Tilt-Relay mounted in this bank. Scroll down in the manual one page - see on Tilt-Relay the switch "3B" - a threebladed make-and-brake switch --- look in the schematic at section / position C-10 --- see that this switch "Tilt-Relay-3B" is in the wiring "making the score-drums to step when playing, making points" ? Greetings Rolf

Does Kinko's even exist any longer? Ours turned into FedEx.