Check your game for cold solder joints. On my Grand Prix it wouldn't eject a ball. The wire looked like it was soldered to the coil but it had a loose solder joint. I had 3-4 other wires on the game with bad joints. Give them all a tug and make sure they are soldered on securely. You also need to methodically go through all score reels and make sure they are adjusted right. Took me a while to get my game running but it has been Rock solid since going through everything.

Congrats on the pickup! Grand Prix is one of my favorite EMs. Just curious as to what you picked yours up for? I paid $750 for mine around Christmas non working but with good cosmetics. Thought at the time I might have overpaid but the game was within an hour drive and now that it's playing great it seems well worth the money!