Before yu do anything else do the ground mods (if not already done): here's the link for John Robertson's site (who first proposed the Gottlieb ground mods)

http://www.flippers.com/gottlieb_ground_cures.html

Note that the 69VAC that feeds the high voltage section (+60 and +42V) is NOT referenced to ground. So, when checking this you need to measure directly across the xformer windings and not relative to ground.