Okay, I've settled down a bit. I managed to get a couple hours to look at this problem without interruptions. I measure the coil and I'm only seeing 7VDC. So I trace the wiring under the PF and there is a 2.5SB fuse practically 12" from the coil. I pop out the fuse and it's blown. New fuse goes in, power up the pin, and I have 34V at each coil again. Great.
Next step, the board comes out. I remove the TIP122 and 2N4401 pre-driver. The TIP122 is replaced with a TIP102. I check for solder splashes and also check for continuity from the top side of the PCB side to the bottom side. I check for continuity from U20 pin 3 all the way to 1J11-1. Everything checks out. The board goes back in. Power up the pin, and the outhole kicker locks. I shut it down before it pops another 2.5SB fuse. I disconnect 1J11 and bust out my logic probe. The readings I'm seeing are almost identical to what I saw before.
*Sigh*
So I put the ball release into coil test mode. It uses 1J11-3 and Q25, Q21, and U20 pin 11. This will at least give me something to compare to. The signals coming out of U20 pin 11 are nice and clean with a nice BEEEEEEEP . Low pulse out of pin 11. Clean pulses out of Q21 and Q25. I switch back to the outhole kicker coil test. The signal coming out of U20 pin 3 is dirty. Instead of being a clean Low pulse, it's a Low pulse, but I can see the High LED ever so faintly and the sound out of the .30 cent logic probe speaker sounds distorted.
I switch the pin off/on again and finally the 4 player display gives me the answer I think I've been chasing;
U51 PIA FAILURE
Well shit.
I switch the pin off/on again and again see;
U51 PIA FAILURE
I switch the pin off, unplug it, let it sit, and then plug it in and turn it on again. The error is gone and the pin boots normally. Heh.
I don't have any more time tonight to continue troubleshooting, but I'm guessing this is why my issue seemed so random? I'm hoping the pop bumper is fed off U51 as well because it's still machine gunning in whenever I go into test mode.