"With everything on sub-boards the first suspects are the card edge
connectors. Take a close look at the sockets to see if the plastic is
breaking down and the body of the edge connector socket is spreading out
at the middle."

Not this time. I understand what you are saying but in this case that isn't it. I did look at the connectors actually and they are that blue plastic. I KNOW that blue plastic is not stronger or better due to its color but I think they have a tendency to only use that color for really good plastic. Thaat is really only an impression I got, but it is based on real world observations.

At any rate, I looked at them pretty good and they are not splitting. And the fact that the new fault came after I sliped with the probe, that ain't it, at least right now.

"Of course clean the PCB edge connectors, etc... "

Been wiping the edge down with LPS2 which is really fantastic. Never ever becomes conductive and really does the job better than I thought it should. Someone showed it to me about 35+ years ago and I have been sold ever since.. I have never ever used Deoxit, I simply have no need for it.