does the mpu board sitting OUT of the pinny measure 5v or 2.4v?

It seems like you're now saying when it's in various machines it's starting to fall below 5v....probably still to do with current-draw straining the cap(s) or chip(s) at fault.

If you're trying it back in a machine, ONLY plug 1 cable back in at a time, so you can isolate which loom (lamps, solenoids, switches) is causing the pulldown effect.