Problems

A- Check your gapping on the contacts for the pop bumpers. Gravity has an influence on switches and while the playfield is up, the switches are probably making proper contact. when down, the gap increases. This can cause missed scoring or "chattering" that scores multiple times. In one case, I had a game that had a broken wire in the harness that when the playfield was up, functions worked properly and when down they did not. The harness would flex when the playfield was moved, but I think it's most probably the switches.

B- check your nine position switch on the score reel