I am running a chevy 350 engine out of a 83 motorhome in a 76 oldsmobile for an enduro car.  After the car is upto opporating temperture the car is really sluggish and the oil pressure drops down to about 5 psi.  When the car is first started it will stay up around 40-50 psi.  Any suggestions what might cause this?  

Sounds like its tired. How many miles are on it?  

Sounds like a spun bearing.

If not, maybe the check spring in the oil pump is bad. You could try swapping out the pump and see what happens. Also check to make sure the sump isn't being uncovered while you're running the car.