Oil pressure problem!!!

Hello all, this is my first post on here. My oil pressure is moving back and forth at an idle. I've had no luck with diagnostics. Engines seems to run ok. However my yellow check engine light will come then my oil pressure goes back to normal but the power is cut down. Any advise would be great. Thanks

 

Thanks yes I did that and it was reading 36 and the gauge was staying there as well. I'm going to have the electrical checked now. Hopefully a wiring harness

 

Just an update on this. Frieghtliner said there is a sending unit behind the drivers seat that sends data from the truck to the ecm. It's showing to be defective. I'll post weather it's a fix or not.