ddec II fuel pressure?

My 93 aeromax has a sick little problem that I have been chasing, no codes and all readings seem normal enough. It's a 12.7- 370-430 flashed to 425 to 1800rpm and 400 from there to 2100. Looses power after you have been in the throttle for a time, about the time you hit the high side on the 9 speed. If you idle it runs ok on take off but seems down on power even when it runs ok. Down a long hill out of the throttle it picks up some for a few sec then fades out. 4:33 gears and it's all it will do to pull around a D3C on a tag trailer, it has been converted to a dump from a road tractor. The boost goes down as it looses power, 20 PSI when it is running it's best and on a good hill down to 10, no smoke so I am thinking it's not getting fuel. I had the head done about a year ago, blow gskt so I had it gone through when I had it off, new injectors and new valve adjusters. It seems to have a miss that is worse when the power drops off. I checked the pressure today, no bubbles in the clear return line I put on, but the fuel gauge jumps, more like vibrates, between 0 and 25 at idle, 25-45 at 1200 and 50-90 at wot. There seems to be good pressure but the shaking needle is hard to read, movers way to fast to see. Also as soon as I shut it down there is 0 fuel pressure on the gauge, I clamped off my return line and shut it down, by the time the engine quits spinning the pressure is 0. Is this a normal thing or is it bleeding back throught he lift pump, or possible o ring in the head? Any ideas?

 

Thanks for the pressure info, I figured the pressure was in the ball park but wanted to be sure, it is a dry gauge. The exhaust has a small resonator and the rest is straight pipe. I added the resonator after I removed an old muffler several years ago because of a crappy mount by the PO. Couldn't take the straight pipe and the resonator fit good under the cab and I can stand to stay in it. Should it hold pressure after the engine is shut down, or does it drop as soon as the pump stops? I will redirect the hunt to a new area, was hoping to find low pressure, would be an easy fix, so the hunt continues.