If i have a similar problem on any B/W machines, I check and clean everything assosciated with the ball trough cause it is likely the game can think a ball is missing because the trough switches/optos need cleaning or adjusting.

The switches will work when triggered by a finger hard, but sometimes a ball rolling accross isnt enough, hence clean, clean n clean then adjust anything u can get your hands on if need be down there. I have also purchased new switched before and it has overcome the intermittent issues.. I just cant remember the exact set up on a TZ, but I am sure it is similar to widebody Indy Jones. Check you ball lock switches etc whilst u are mucking around if it it easy to clean n also check.

If anyone has anything else to offer, they will jump in for sure n post if I have missed anything or stuffed up anything in my advice.