Well incredibly the problem has disappeared! I had taken out the U20 & U50 out of the board, sprayed the sockets with IPA and then I placed the PG into a TM502A and while monitoring the 5V rail left it to it's own devices for a considerable period of time. I had noted some initial occasional voltage excursions well north of 5V but it never got to the point of triggering the crowbar. I then decided to put the PG506 back into it's TM505 home and left it for hours. Not only did it not fail but I noted that some previous amplitude jitterings on the fast-rise pulses had disappeared.
Since the U20 also provides the Vref for the 5V rail via U50, it would seem that these jitterings were likely the original cause of the random 5V failures..... fingers crossed but the PG506 is still running and.... NOW.... I'm going to do the 2467 CAL (out of the fireplace and into the fire!).
