What do you have for lines size in and out? I have the same pump on mine 94f250 and i can maintain 6.5 pounds all the way thru to 5500prm. I run a #8 (1/2") feed line and a #6 (3/8")to the carb. Just wondering if you have a crimp in the line or break in it. I was have issues like that on my bbf in my 74 f100 with a stock pump. Truck would idle nice and would open up under no load, but under a load, fall on its face, Ended up being small crack in rubber line by the switching valve. IT was sucking a bit of air.