Losing oil pressure after engine reaches operating tempatur

I have a 1996 dodge 1500 ram 4x4 with a 360 and 183000 miles on it. I had some problems with losing oil pressure after pulling my camper for awhile. I replaced the main and rod bearings and the oil pump and the pressure came up. About 3 weeks later my oil light came on after driving about 10 miles, but the gauge stil read about 40 psi. I installed a mechanical gauge and the oil presure at idle while cold was 70 psi. So I figured my sending unit was bad because the gauge never read above 50psi. I drove home(10 miles) and had with it in gear the mechanical gauge read just above 10 psi. An hour later it was at 25psi. Any ideas where to go next.
thanks,
matt