OK, enough suspense....I don't anyone to blow an artery over this.
The pan had what appeared to be a relatively minor dent from the
outside......however, internally, it completely closed the gap between the
pickup and the floor of the pan. At any kind of engine speed, the pump
would try to flow more than the impeded pickup could provide, and Presto!
Inverted oil pressure syndrome. RPM go up, oil pressure goes
down........RPM go down, oil pressure (now fully fed) goes back up to where
it should be.
I can't imagine how many "mechanics" I would have had to go through to have
this diagnosed if I had taken it anywhere, rather than fixing it myself.