Oil Pressure

Hey all, I got a problem with my 318. I have 55psi on start up, cold. Once engine is to operating temp. I have only around 8psi at idle. I checked it with a hand held fluid filled guage as well as the one I installed, and it's the same, so that rules out a bad guage. Running down the road at 1800 rpm it's at 10-15psi. I have the engine out and have rechecked the bearings, all OK. I just put this together about 1000 miles ago. New rings, mains and rod bearings, cam, lifters, springs, valve seals, timing chain, oil pump, etc. Just a basic overhaul. I did compression test and all cylinders at 130-135 cranking the engine over 3X each cylinder. Have oil at rockers, no noise, but I do have a slight knock at idle when the oil pressure is real low. Bad pump? Any other ideas?
Thanks.