ok ... so I decided to swap the power driver board with a freshly serviced one,
and leave unplugged various circuits and disabled the gumball and clock.
The U20 chip and F115 fuse, died after 3 games.
So the serviced power driver board is not at issue. Still same CPU at this point.

My friend and I spent over an hour inspecting various switch areas for contact issues. Not find any recognizable.
The one area touching is the inlane switch mounting base touches the underplayfield support metal, which is grounded but I believe that would make no difference since there is no ground row short.

For my playtest, I left unplugged:
- J120, J121 for the GI ... so does that mean conclusively that there is NOT a short between the GI and switches? I would say yes.

- J138,136,135,134 ...all related to insert lights ... so this feature is not at fault. correct?

- J125 which controls some of the flashers, motor and loop magnets.
(J124 was plugged in as I saw some flashers and the mini powerfield magnets worked.)

It happened during regular multiball ... no balls were on the mini powerfield.
Oh, and I also restored the 2 standup switches that I forced to be stuck closed (this had brought the average voltage down to 11.5ish like all the other white wire rows.) So the white/grey was back up to 13.7v and would drop to 11.5ish with one switch closure.
No fun!

@zaza I will try more to inspect the 12vUnreg shorting to the white/grey wire...it makes sense.