97 Neon DOHC Oil pressure-Relief valve or bearings??

97 Neon Low oil pressure after it gets hot. Installed a mechanical guage. When it starts, approx. 50psi. then drops to approx. 20psi after about 1 or two minutes. After driving approx. 10 miles, it runs approx 25psi @ 3000rpm/65mph, it drops to 6psi while idle. Does the same with 5-30 and 20-50 oil. Changed rod bearings, no help. Does this sound more like a oil pressure relief valve issue or main bearing issues?? It appears the previous owner replaced the oil pump to try to fix the problems. Motor runs good, no knock or ping. Any help would be appreciated.