My van loses oil pressure after it gets to operating temperature

What is your question?

I recently purchased it put a rebuilt computer in it to get it running the van sit for about a year before i purchased it. Ive been told that the pcm could have something to do with the problem. Ive put some marvels mystry oil in it thinking it might have some sludge build up in the screen. I took thevalve covers off only to find motor fairly clean for 85000 miles can u give me some ideas with this problem