Had this problem on a couple of games. Check the 'ball jam' opto - it's located between #1 opto in the trough and the shooter lane to detect if there's a ball stuck there. The game is seeing the opto as closed so it keeps kicking balls out to clear the non-existent jam.

In one case is was a bad IR emitter; replaced it and no problem. The other case (and this took me forever to figure out) was a broken wire in the harness around mid-playfield. It was on the same line in the switch matrix as the ball jam opto. Figured that out when there were two other standup targets that weren't registering as well.

Good luck!

You can do a quick check of the IR emitter by looking at it through a digital camera. It will glow like crazy if it's working.

If it is dark, it's probably just needs to be replaced. This is pretty common since the trough gets a lot of vibration. You should also check continuity between the emitter and it's next connection just to be sure it's not a cracked solder joint or broken trace.

If it is bright like the others, there could be a broken wire somewhere after the trough board. It could be the detector but it rarely goes bad. Doesn't hurt to check that side for solder issues as well.

I'm not sure how JY is set up but *sometimes* there is an extra IR emitter already on the trough board that is not used. If your soldering skills are decent, you can steal that one.