Reflow your solder joints. See image from the SBM that I recently worked on.

https://pinside.com/pinball/forum/topic/polishing-up-a-silverball-mania/page/8#post-4227422

pasted_image (resized).png

if you isolated the problem to a specific MPU you want to be checking the lamp address and lamp data connector pin solder joints leaving MPU j1. Most likely the connectors, but could be a bad PIA at U11 or the U11 socket is flaky.

Does the switches work properly on the problem MPU? Shorted switches can show up in the lamps because the same PIA ports service both.

When you say "connectors are good" do you mean the connector pins are NEW or 40 years old?

Those 40 year old pins were only designed for 20 installation cycles. You know by now that number has been greatly exceeded.

Every body hates hearing this, but the only way to insure a totally reliable Classic Bally is to repin both the male and female connectors.

Sometimes even the weight of the wiring harness causes connectors to open and close.

Save the female plastic housings, as many are no longer available.

-

How about a picture of the PIA sockets, without the chips installed?

Some brand of sockets are notoriously bad, like Scanbee

Did you lose the battery?