My car has been sitting for about two weeks. Today I started it (easily) and drove it at speeds less than 50mph for about fifteen miles. At about the ten mile point, the oil pressure light began to come on and go off intermittantly. There is plenty of oil and it was changed less than 3K miles ago. Temps here are about 40 degrees. Help!

 

It's either a problem with the sending unit (cheap) or a more serious problem (expensive). Get it checked right away. If you feel up to it, replace the sending unit and see if that cures the problem. It's on the front side of the engine toward the passenger side of the car, accessible from underneath.
If that doesn't take care of the problem, it may be a result of a worn oil pump or badly worn engine bearings. Those are major problems.