Agreed, but those optos are hard to find. I have an old TZ clock board I scavenge from.

Although that is usually the problem, it can also be related to the flipper boards for some min PF's, such as this and more commonly TS. I would first try swapping the flipper opto boards, if the issue remains unchanged, it's time to dig apart the left rear and examine that board, could just be cracked solder joints on the optos that need reflowing if the problem is intermittent. If the problem changes to the other side of the mini, it's a flaky opto on the flipper board.

As I noted, you should have tried swapping the flipper opto boards first. It's likely not the issue, but it could be, and is an easy first test.

It sounds as though you are not experienced in fixing these (just by what you posted above). If so, I would recommend posting back on taking the mini PF off when you get that board, as this can be a little tricky, unless someone has a thread detailing this. It's not overly hard mind you, you can do it, but it is more complicated than most things, and that mini really has to be levelled out after for the left and right action to remain accurate.

Those small board optos don't register in switch test mode.

Are you good with reflowing solder joints? Reflow the optos and try the board again. If it's intermittent, that means the opto works, but there is likely a cracked solder connection. If it doesn't work at all anymore, either the opto is bad, or the solder crack is allowing now connectivity. Again, reflow, see if that works, and if it's still intermittent after this, again, try swapping the flipper opto boards just to be sure.

PM me your number, I'm in London, I can give you a call tonight or tomorrow and try to help you work through your issues if you want. Sounds like you have a couple things mixed up.