ISX15 2350 oil pressure acting strange

Just got truck back from rebuild. Was test drove and was good to go. I bobtailed home...everything was good. Oil pressure up around 40-45 at highway speed @ 1500rpm.
Hooked to a tanker today and drove about 15 miles and suddenly my oil pressure was at 30psi. Pulled into loading facility and stopped at office. Started driving and my oil is at 40psi again.
Got loaded and drove about 10 miles and it's down at 28psi again (60mph @ 1500).
Pulled into fuel station (15psi at dead idle) and popped hood and everything was dry. I checked the oil and it was good. Started up the truck again and now it's at 25psi at dead idle and back to 40-45 at highway speed.
It continued to do this for the next hour until I got unloaded and got home.
Then I bobtailed back to the mechanics shop (about 25 miles)...and oil pressure was perfect the whole time.
I was thinking oil pressure sensor but why doesn't it do it when I was bobtailing? Oil pump (shimmed up) and oil cooler both new with rebuild.

 

Yes all new internals except crank. New head, camshaft, injectors, rockers, cooler, pump, and did front structure at the same time

 

Thanks. It's sitting outside the shop now. Will be going back in tomorrow. He did order a sensor but it's intermittent so it'll be hard to tell what fixes it...unless he can see something in Insite. I'll mention to check the wiring where you suggested though. Thanks!

 

So I got the truck back yesterday. Of course it runs fine in the shop so putting it on the computer didn't really show anything.
They did change oil pressure sensor. Also (when I did the rebuild) I sent injectors in for testing. 2 passed and were re-used. But one of those 2 are returning back too much and causing an spn 157.
Today I took the truck out hoping the leaky oil sensor had been the problem. It wasn't.

I did find out the following yesterday:

It runs at about 40 or so until oil temp hits approx 212-213. As soon as it hits that temp....you can watch the pressure drop off to 28psi.
It doesn't change as you drive...even if oil temp drops to 200F...it will stay about 28-30.
BUT...if it drops under 210F and I make a stop at stop sign and then take off again....the oil pressure comes back up to 40+.
As soon as it hits over 212F...it drops off again. It does NOT throw any codes when the oil PSI issue happens.

Also, now I'm getting code 95 instead of 157 while accelerating loaded. Injector hasnt been replaced yet...so that's probably where that code is coming from.

I'm going to try changing oil filter... just in case. It's a fleetguard on there right now.

 

Yeah we shimmed up the pump to run a bit higher pressure. I read the new style pump is less pressure and we run heavy weight so I wanted to at least have what I had before the rebuild.