07 9400, Cummins ISX engine 8520 CPL Oil Pressure Issue

I am having an issue with the oil pressure acting erratically. My driver called and said the pressure had dropped to 20 pounds running. He was close to a shop and it was time for a service, so I had it serviced. it seemed ok, but when he got back I noticed it bouncing around. 40 PSI cold and when I revved it up, the pressure sent to 55. warm it is usually running 35 psi give or take, but I saw it going down to 20 running. idling at 20 and sometimes down to 15. I have receipts for the truck getting a bottom end done, including a new oil pump. Anyway, I replaced the oil pressure sensor and oil filter bypass valve with the new style. No change. My cummins buddy is going to hook up a gauge manually and see what we get. He also said, in rare occasions the spring in the oil pump breaks. Then he threw possible cam bushings, which I didn't want to hear. Also hooking up the computer to see what the ecm is seeing for oil pressure. Anyone have similar issues?