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I ran a engine cleaner through my 350 in my 93 truck. About two weeks later, my oil pressure gauge went out. I took it to the dealership and they told me not to worry it was just the guage was bad. Ive since replaced the guage and the oil pressure was not very good. 5-10 at idle. I took off the pan and replaced the pump with high volume and pickup screen. I now have 35-40 lbs at idle when its first started. But after it runs for a few minutes, the pressure goes down to almost nothing and the engine starts knocking. It stops knocking when I rev it up, but when it idles down it starts knocking again. Restart it after it completly cools down, it purrs like a kitten, until it warms up, then zero pressure. Does anyone have any ideas on the cause and the fix?  

Bad main bearings  

it does sound like it could be a clearance issue.  When your car is cold your oil is much thicker then when it warms up it becomes thinnner. If you have worn bearings, there might be to much clearnece to create a significant amount of oil presuure.  

I had the same problem with a 99 surburban. Turns out that the mains were not the problem. The cam bearings were worn and this is what caused the low oil pressure. What you said about your engine after putting in the new oil pump is the very same thing that I had. It had better pressure cold after installing the new oil pump. Once it warmed up the pressure went to nil. Good luck.