[Rob Petrie E395]

A couple of things come to mind after reading your post.
1. Have you tried a different oil filter. I know it sounds crazy but there are people who let a car sit all winter without changing the oil and filter. If the filter was collapsed and you never changed it just pumps well you get the idea
2. Did you remove the bypass spring before you brazed the pickup on? If it gets too much heat while brazing the inlet on it will get weak and can cause low oil pressure
3. Define pretty close. If there is a leak couldnt it pull air instead of oil? I could be wrong but I would think it would be easier to pull air through a gap than 20W50 through the pick up.
4. How old is the engine? could it have been built with loose bearing clearances and now that its got some wear on it its way too loose. Sounds kind of like when the oil is cold its ok but when the oil gets hot and thins out a bit its loosing oil pressure couldnt that be a sign of too much clearnace somewhere.
I am no engine builder by any means but thought I would throw it on here. I am sure if I am a idiot someone will tell you in a minute. LOL