I hope you got it fixed. If not, I have one more thing to offer. I had the same thing happen on a Williams Flash. It turned out to be the metal leader going to the lamp in the pop bumper. They have a clear plastic tubing over them. I guess that the previous owner had replaced the lamp socket and forgot to put the plastic tubing back on. The lead was very close to the mounting bracket for the pop bumper. Sometimes when one of the pops fired, the game would tilt or get some crazy switch error. Replacing the tubing fixed the problem.