Do you have a schematic for the machine? You would need to trace the bonus countdown circuit to see which switch/relay is out of adjustment. Also, it might be an electrical short, loose wire, etc. I would start at the outhole switch, because that's how the bonus countdown is activated. Also, manually operate the bonus unit to see if it will reset and/or countdown properly. You can post a photo of the bonus countdown circuit if you need help on what switches and relays to look at...

I've never worked on one of these. From the pictures, I assume the bonus unit is under the playfield. It doesn't step each time a target drops? If not, then somehow it has to know how many targets are down.

I have a Solar City.

Sometimes the ball will enter the ball return area fairly quickly, will bounce, then roll through the ball trough. The "Ball Trough Switch", will change the game to the next ball or next player. On the 5th ball you will get the never ending game. Some owners have placed thin foam in that area to stop the ball from bouncing. The foam is not visible from the players position. Now if you have the game set on a steep angle, with fresh wax, the higher ball speeds can create the ball deflection.

Next:

That 'Bonus Unit' is as EMsInKC states is attached to the underside of the playfield. As with any Gottlieb with these steppers they do fall out of adjustment. Does it usually count down the bonus points when the ball lands in the ball return? Ball lands in hole, then you wait as it steps through all 15 contacts, no fun when there are no targets knocked down.

This solved the bonus problem I was having ,see if the bushing on the bonus stepper has free play

https://pinside.com/pinball/forum/topic/high-hand-bonus-scoring-problem-solved

Same problem on my Solar City - bonus works perfectly 90% of the time. Then randomly a ball drain fails to initiate the bonus collection function. I don't think it is bouncing through the trough, but I will keep an eye on it.