i am thinking a air leak is hiding somewhere to give you this problem
Was wondering the same thing. I bumped a line in my cab lift/mount replacement and it crumbled. I took it out and it looked like it had been chafing against the frame and it did leak. It was the return line, never lost any fuel, until I bumped it, but I replaced both lines from the pollack to the front tank and I swear I picked up 15 HP! Truck ALWAYS started easy, etc. until the line gave out in the swap, but even then, it would fire off, die, then start and run, albeit lopy. I pulled two 1000+ lb beef cattle and trailer to the processor 50 miles away and home empty and it never missed a beat, even with the gaping split in the return line, but it was tough to get running after it set for any time.