1999 Cadillac Deville high or no oil presure

I bought this car with blown head gagskets, replaced them. Experienced low oil presure fault shortly after rebuild, put gage on and had 70 psi at idle. Replaced sensor. Shortly after low oil presure fault occurred again, this time no oil presure. Found balance wheel bolt loose, tightened it, could not get oil presure, filled pump with vasaline to prime up. Obatined 70 psi at idle. Drove car 2 weeks. Lost oil presure while driving. Towed home, took pump apart. Cannot see anything wrong. Hooked clear plastic line on intake port of engine block and vacume easily sucks up oil, no blockage in screen. Took oil filter casting off. Passage clear to filter neither relief valve in punp or filter casting appear to be sticky.. Put back together and have 70 psi at idle. Drained oil out filled crankcase with diesel fuel. Ran for 6 min. Oil presure at 20 psi. Changed oil and filter. Presure at idle was 50 psi. 100 if reved up slightly. Let engine cool. Started up, oil presure 80psi at idle. I am afraid of blowing the oil filter off, or losing oil presure altogether again. Any suggestions?