Ok...now Im asking for help...

Rebuilt engine on 87 vanagon (changed head gaskets, rings, all gaskets
and hoses, new plugs, wires, cap and rotor)
Car will idle fine for about 15 minutes, then oil light will start to
flash
Everything seems to work and run fine except
now my oil light and buzzer goes on at about 2200-2500 rpm
and then kicks off above 2500.

Picks up fine and never seems to want to die no matter
how long I continue to drive it with the buzzer buzzing,
but of course, I never want to drive it very far after it goes on.

Question is...
        I was not having any oil pressure problems before the rebuild.
Should I be concerned about the oil pump?
I am planning on changing the sensors and want to install an oil
pressure guage. Oil level is normal, no leaks.

Car will idle fine for about 15 minutes, then oil light will start to
flash.

Anyone with suggestions?
Thanks in advance...