Dirty optos are common, so clean them. (I'm sure you did this...)

A lot of opto failure is found when you unsolder the QED123 (transmitter) or QSD123 (receiver) LED's. You'll find that one leg is actually broken, right at the top of the board, still making connection... until it doesn't.

While a lot of games this age are suffering capacitor failure on the opto boards (there is a radial 100uf 35v capacitor that is corroding traces and causing random opto or other switch matrix problems), on your game I think the opto board has an axial capacitor, and those seem to not be failing...

Anyway, check the opto board and see if there is any hint of corrosion or a 'wet ring' under the capacitor. There won't be, but check it anyway.

This could be quite hard to find, so try to force a failure by going into switch edges test, and bang on the VUK mechanism underneath with the handle end of a screwdriver to see if the beam is getting disconnected intermittently by vibration.

Other than that, test every switch on the playfield and see if there is any phantom triggering due to a diode on some other switch not properly protecting it's row/column.

Good luck!

Agree with everything PinRetail said, especially check for dirty optos if you haven't and clean both sides the transmitter and receiver. Aside from blocking them physically, you can also look at them with your cell phone camera to make sure they are working. They aren't visible to the naked eye, but they show up great on your camera. You will also want to look at connectors on the opto board. Check the wiring and the connector header pins coming off the board and see if any have gotten loose or have cold solder joints. Also, not just test every switch but examine every switch and make sure there isn't a loose wire on all the switches in that row and column. Putting the game into switch test mode and just pounding on the center of the playfield and seeing what comes up sometimes helps too. Sounds a bit like banging on an old TV but I swear I have solved plenty of switch problems by starting this way. Good luck and let us know what you find!

Hey!

I was going to suggest that also!

It might just be the opto need to be replaced. They can break for various reasons, including the stiff wire coming out of the head of the led. Lots of places sell them. To verify, you can swap it out with another one in the game and see if that fixes it. Just swap out the emitter. If the problem continues after that, swap out the receiver as well.