Quoted from yellowghost:
If brand new optos,then definately a bad solder joint or loose wire. I would say goto switch test and rattle the machine a bit. I read on here about a person turning on a shaker motor with switch test and revealed that.
One other thing . I had a opto problem with a different machine and checked everything. Turned out to be corrosion on cpu board. Worked fine after I cleaned it. If you have spare cpu board,swap it and see what happens . Can't hurt
I think it could be a loose wire or something not soldered all the way as well. My optos were very finiky until I reflowed/cleaned them (I know you replaced them new though)
I wonder if they measure a solid 12v when using the upper PF though, (maybe you can meter them while someone controls the flippers)
One last thing I had where I would get random fires - It turned out that the kicker arm was not fully retracting 100% of the time for me. What would happen was it was just enough that the yellow piece would still somehow break the optos from time to time. I ended up replacing the coil sleeve and it never happened again (turns out the sleeve was chewed up a bit on the inside because the whole thing was loose to begin with.)
Either way, don't give up man!