OK, on diode test, LM339 tests exactly the same as a working board (pins 1 6 and 7, as well as pins 8, 9 and 14)

Replaced it anyway, still no change.

I am starting to think it has to be something to do with the grounding of the 5v regulator, since sometimes i have deliberately put a resistor between the regulator and ground to up the output from 4.8 to 5.0V (the regulators are still within factory spec, but just aren't up to the task)

Do people think it is normal for the voltage to jump from 5.2 to 5.5 or 5.6V as soon as a load (ie: plug in J114) is placed on the board. I don't think i have seen this before, and does it offer any clues?