Turned my WOZ on and the balls kept firing out of the trough and autoplunging themselves. The throne room VUK was firing and the ramp ball lock was going up and down. I can't remember if the winkie drop target which is also on this circuit was firing, likewise the ball divertor.
I opened the coin door to stop them, and noticed they were all on the J106 orange wire circuit. This is associated with fuses 701 and 706 which I checked (pulled them out and tested) and they were fine.
The trough VUK ended up with the black tip falling off.
I have checked for obvious things like whether there is any metal touching the coil lugs and haven't found anything. Just waondering what the next step. I'm a bit rusty on testing transistors but I would be surprised if they are the problem seeing as there are a few different coils on that circuit firing.
Just turned on the machine again after a break and now just the Auto-launch and the ramp ball lock are firing. The throne room VUK and the trough VUK have stopped.
It's a RR75 LE, built late 2014.

Thanks, I forgot to mention, I did go into switch test and didn't find anything untoward. And one bad switch wouldn't affect more than one coil would it?

Just to report back on this...thanks for your answers you were all right it is an opto problem. Column 1 and 2 in the switch matrix were green, which I assumed was good, but of course is bad (there were no balls in the trough at this time).
I have actually fixed the column 2 optos by wiggling the connector at J202. A bit dodgy as a bit more of a wiggle and it could go back to bad again, not sure how to fix it permanently.
With column 1, the trough optos, they are still out. I have reseated the connectors at the trough, at the back of the board chassis and at J200. No change. Not sure what to do next...

Problem fixed!
A thousand thanks to Bud who patiently took me through it on the phone. It turned out the connector at J201 was also a bit dodgy and reseating it and wiggling it a bit got the trough switches back online.