Oil pressure light on

I have a 95 eclipse gs non turbo with a bit of miles, 145,000. The engine locked up and I did a rebuild with new bearings, crank, piston, rings and valves and replaced oil pressure switch. When at idel when the car and oil warms up I lose oil pressure bad, then if I rev it it regains pressure but I have to keep the rpms higher as the engine heats up. Any idea whats causing this? My first guess is oil pump but a mechanic told me he doesnt think its that.