The Groundmod is not that importent on Gottlieb System 3's.
If there was a problem with the ground the whole game would reset.
I think you probable have cold solderjoints on the opto leds.
Don't know about your soldering experience, but try and reflow the solderjoints.
System 3 is maybe the most durable pinball technology ever made. The reason that there is so little info is just because these things never break down. But if they do, they are almost impossible to fix.