Oil pressure problem!

[Hawssie]

I have a 1970 c10 with a sort of crate motor in it. The motor was one of those newer design 1 piece oil seal 330 hp crate motors GM dealerships used to sell a lot of years ago with the then new "VORTEC HEADS" on a carburetor motor. This crate motor had a warranty problem and was replaced with a new one, in which time the parts counter guy ended up with it and had it fixed by a machine shop that did work for the dealership. He was going to use it but ended up trading it to me. This all happened about 10 years ago. I had the motor sitting around for 9 years and last year I put it in my 1970. Well I set it up and the oil pressure was fine as I remember but I never really drove it. That was about 8 months ago. Now I am back working on my truck and I took it out today for a 10 mile drive and I noticed the oil pressure was going to about zero at times. I took it home and put a mechanical gauge on it to confirm. and it seems to be around 25 psi at start up while warm. If I rev it up it will build to about 45 psi tops but after I take my foot off the throttle the oil pressure starts to gradually drop and bottoms out at 5 psi and sometimes lower. I checked the oil plenty of flow getting to the heads.No drivetrain noise. I have never had symptoms like this. It also sometimes jumps around a bit at times. What sets the oil pressure? I know bearing clearance problems will lower it but could it be in the oil pump maybe? Is it time for it to come back apart?