Thanks to the list-- and particularly to Harry Schwartzman and Hilary Stone-- I am pleased to report that three layers of rim tape worked with my m58 style vintage Mavic rims. (Specifically, two layers of narrow tape-- Grand Bois cloth and then one of nylon-- within the rim- well over the spokes, and then a wider nylon strip on top of this, slightly raising the floor of the bead seat.)
The problem seems to have been fairly simple-- which is to say, the tire was twisting slightly under pressure so that one side could slip slightly lower down the rim wall and allow the other side just enough room to slide up and over the rim bead on the other side. The solution was making the rim seat shallower.
Thus, I'm now easily getting the Panaracers to hold 80+psi without any sign of problem And on a gentle 7 mile spin yesterday and a slightly brisker 10 miles this morning all was exactly as it ought to be. Basically less than an hour of work was involved and about $8 worth of rim tape required.
What can I say except I give ALL credit the CR list for making it crystal clear that the problem really was extremely abnormal and could only be caused by one of 3 or 4 factors-- each of them fairly improbable.
As for the rims being slightly undersized-- for a while at least the only plausible theory-- it is worth noting the wheel builder (a list member) pointed out that he'd almost certainly have noticed this when he was cutting spokes.
Accordingly, I can now join those of you who are enthusiastic about Panaracer, and very happily retract my highly critical comments of yesterday.
Once again, my lasting gratitude to all.
Guy Lesser
Brooklyn, NY