90 astro/oil pressure problems

well now got an astro 90, 4.3 rebuild bored out .040 and after 6 months i notice the oil pressure guage bottoming out, check the oil its ok,,,,,now when i start it the guage is right where it belongs, then after about 5 miles come to a stop and it hits red, have to rev it a little and it goes up a little. hmmmm was told change the oil pump, the one i put back into it had a nylon shaft to the distributor, both mech. i talked to said dont worry about putting a high volume pump in, you wont have the clearance. well i checked the grounds cleaned them and even seperated them since i had two on the same stud. any idea's will be welcomed, and one more note, when it hits red on the guage, i get a clunk clunk clunk noise , hopefully its not a main or a connector rod going.

I had a bad rebuild once that gave the fault you have. Good cold, bad hot. I found that the main bearing clearances were too big, and that the pump was fine.
What to do ? I'd try some 50 weight oil and see if the pressure stays up. Not sure what to say about checking stuff. Does the oil pan come off without lifting the engine? If it does, you could check the bottom end, (plastiguage), and have access to the pump, if the bearing are good. But that still leaves the cam bearings suspect.
Hopefully others will write in. good luck

I sure hope I'm wrong, but...I bought a '94 w/4.3L that was on it's third motor. The bottom end had gone out on the previous three so I was suspect..anyway, after a few month lost oil presure just as you've described. Pulled the motor and found all the main bearings where shot and the rod bearings where getting rough. When I went to pickup my reman'd long block the rebuilder said the warranty would be void if the old oil cooler was reused - he had one under his bench that they'd disected. The oil passages are so small that they retain bearing bits from the previous disaster only to break your heart with the next motor. A former GM parts man looked at my old radiator and confirmed that it was the factory original.
Why was your motor replaced - bad bottom end? Use tin snips and cut open the oil filter, see any metal?
If it's knocking it may be time to air-up the bicycle tires. Sorry.

I have the same problem with my 95. The first time I noticed the gage go down to zero after coming to a stop I checked the oil level and found it a bit on the low side. So I added a quart. This seemed to remedy the problem for about a week than it was back to dropping down again. This time I changed the oil and the problem went away. But only for about two weeks now it's doing the same thing again. But unlike your problem I don’t hear anything when it happens. Is this something to be concerned with?