Just turned on my STTNG and the canons just kept cycling round and round until they faulted out. Went into test mod, they test fine for switch activation showing closed and closed when reaching the park position and showing open and open when moving off.
Also found that if you press play button, it will pop all of the balls into the launch area, crowding them up on top of each other. Again, go into test function, check all trough switches and ball launch switch, all working perfectly. Drop a ball down the hole, and watch them all get switched as the ball passes.

Checked all ribbon connections, as have encountered that before. All seem ok, plus the tests work perfectly.

Any clues most welcome.

Thanks for all the suggestions. Here's the results.

Opto's checked in canons and trough (replaced with Great Lakes mod a year ago) with camera (to see them working) and with switch test and finally with good old fashion finger in the front of them test and all working fine. There doesn't appear to be any opto's self firing over a five minute period.

Looms checked with test meter and visually for shorts and continuity and all perfect (were replaced around nine months ago).

Canons don't fire whilst turning.

Thanks for the link Daley, I haven't had a chance to read through that yet. I'll look tonight when I get home.

Cheers all.

Thanks everyone for the suggestions.

It turned out to be a bad pin contact at connector J3 on the opto PCB under the playfield. The pin was loose and was not making a connection causing the mayhem. Took a while to find, but soldered up now so hopefully shouldn't happen again.

Cheers all.