<VV> Low oil pressure in a late model

[Edelstein and Payne]

Folks,
     Yesterday on the way home from high school, the temp/oil light came on in the vair that my son uses as his daily transportation.  He pulled to the side of thee road and called me, saying the car over heated.  I explained that it doesn't have a heat sensor (it's a 65, 500 coupe, 110 powerglide car) but the light meant that the oil pressure was low.
     We then got some parts and did a number of things in the parking lot where he stopped the car.  We changed the oil filter --- it had only been on there for about 500 miles, but it's a Purolator and I know some folk have had problems with those.  Started the car and the light went out for maybe a minute, then came back on.  We then swapped the oil pressure sensor for a used one that was on the shelf in the shop.  The light stayed on.
     Each time we fired it, the motor ran smooth and quiet.  It sure didn't sound like there was no oil pressure.  I mean with no pressure, the lifters would have been "clattering" like crazy.  So I took a chance and drove it very gently and slowly about 8 miles back home.  (Makes you uncomfortable as hell to have that oil pressure light glaring at you as you drive.)  It seemed to run fine and got home with no problem.
     Found a brand new oil pressure sensor at our local Advance Auto, and installed it.  The light stayed on.  I checked the conductivity of the wire that connects to the sensor, as it is somewhat frayed.  Touching a probe up into the connector, there is continuity to ground, which is what I think it should be.
     After we installed the new sensor and the light stayed on, I decided to rev the motor a bit to see what would happen.  Revved it to maybe 2000 rpm or so (this is "by ear", there is no tach), and the light flickered out momentarily.  Revved again and the same thing.
     It appears that there is a low pressure problem.  So what are the likely causes and fixes?

                      Thanks,
                     Travis Payne   65 Monza coupe
                     Raleigh, NC    65  500 coupe