Watch the action of the 100 point score reel as the 500 points are being scored. The reel should consistently and solidly step 5 times in a very regular tempo. If the reel sometimes doesn't step, or sometimes steps 2 numbers at one time, then you either have dirty switches at the score motor 1A position or you have score unit mechanisms that need rebuilding - or both.
Since the score motor switches are easier to tackle than the score units, you probably want to check those first. Lift the playfield and watch the score motor switches as you trigger 500 points. On one of the switch stacks that rides the bottom cam, you'll see sparks as the switch contacts open and close to rack up the 500 points. These are the contacts you want to clean and adjust. It's a good idea to tighten down the switch stack screws before making adjustments to the switches.
- TimMe