Ram of Steel

I think I may have / had a fuel pressure problem. While empty, the truck runs great. While towing 40 ft 5th wheel in the mtns, runs ok until I get the engine load up. The boost gauge will fluctuate violently between 20 & 30 psi and I basically loose all power.
I installed a new overflow valve today and cleaned the pre filter and attempted to check my fuel pressure. I checked pressure at the P7100 inlet. All I had to check it with was a new oil pressure gauge and a 1/8" nylon hose. The gauge fluctuated so bad, I could not get a high and low reading. It did appear to have approx 20 psi at idle and 30 psi @2000 no load. After about one hour of driving with the test gauge installed, the gauge took a dump.
Any opinions or advice?

Thanks.

gunracer1

put a smal needle valve in the line or use a liquid filled gauge. my fuel pressure reads fine at idle, good down the road, it dosen't go south till i hit full boost. then it drops to 0.