Quoted from PinRetail:LTG has it right.
When this game came out, the plastic flipper button opto interrupt was too... plastic. Replacement white colored interrupts were released, but they had problems with reflections (and transparency?), so the current replacement flipper button opto interrupts are black... the same color as the factory ones that had problems.
Clean the optos while you are there (I use windex on a q-tip).
The progression of this problem is that the interrupt doesn't bend... quite... back into position. It stays just barely on the margin of half blocking the opto, and if the game thinks the opto is un-blocked, it activates the flipper. The interesting thing is that this can cause the coil to be partially activating with a little electrical noise, kind of a very low-level buzz, that causes the flipper coils to get hot to the touch without raising the flipper bat.
A fraction of a degree cooler and the plastic snaps fully back, so the problem goes away when you cut the game off for a moment. It definitely goes away when you open the game to check out what might be causing the problems, because you are releasing trapped heat.
Now this COULD be a flakey opto on the board, but it is far more likely that replacement interrupts will fix your problem:
https://www.marcospecialties.com/pinball-parts/03-9001
I've only seen this on games that don't have a spring steel piece that helps the opto spring back, so if you have spring steel on both flipper buttons in addition to the plastic interrupt, then I'd suspect the opto boards themselves. LTG's suggestion to swap left to right will show you the path here.
This is all super interesting, and makes sense. Thanks for sharing!
