With the power supply replace the PC2 optocoupler, along with the capacitors on the 5V rail again. Your capacitors could be drying up, and the optocouplers probably have degraded as well. Put a scope on the power supply secondary 5V rail and see how much noise there is there, more than around 200mV or so means those capacitors are dying. Have you also changed that main primary side capacitor C5, as that going low value ( or high ESR, especially if it is original) will put a lot of ripple onto the output rails that the controller will be unable to correct.
The capacitors are good quality; they are Panasonic FM series, rated for 105°C. For the big C5 capacitor (330 uf, 200v), I bought a Nichicon, because Panasonics of that value were too tall. I wouldn't expect them to be bad after only 8 years. Most people who have these PP-1000A power supplies still have all the original capacitors in them from 1984.
I don't have a scope.
Something strange happened earlier today. I took the power supply apart to take some pictures of the PCB (I had adjusted it up from ~4.9 VDC to ~5.1 VDC about an hour before that), and I brushed some of the dust off, and when I put it back together, reinstalled it, and turned on the game, there was no music. I know from past experience that that means the voltage is too high. I checked it and it measured 5.45 VDC, and I had to turn the pot quite a bit to get it back down to ~5.1 VDC where it's happy (which brought the music back).
The knob for the pot is flush mounted with a screwdriver slot, and has a fair amount of resistance to turning, so there's no way I turned it accidentally when I had it apart. About an hour after adjusting it to 5.09 VDC, it was down to 4.99 VDC. I suspect that pot is causing the problem. I think I'll order
this one, and if that doesn't fix the problem, I'll look into those other things you mentioned.