When truck warms up check Gage list comes on and I lose oil pressure, it goes to nothing, then back up to normal. Any ideas, thanks
By the way lol, I have a 2001 5.9L gas engine, 2500 SLT heavy duty 4x4 with 290,000 miles

 

Could be two things. Bad oil pressure sensor or (this is the likely cause) your plenum is blown

 

I looked down in the throddel body, the top is quite dirty and black, the inside only has traces of black spots except a place on passenger side. But it's not oily just a bigger black spot.

 

And it's the original plate, I checked it worth small magnet

 

​​​​​​ Only real way to check is a vacuum gauge. Mine didn't look bad but it was blown

 

Replace the oil pressure sensor. That is a famous failure mode for them.