I have a strange issue with one of our work trucks and I'm not sure exactly what the culprit is but here is the story:

2006 4.7 QC 2wd 100k mi. Truck started cutting up so I replaced the fuel pump which fixed the problem. BUT I ordered a regular pump and when I pulled the factory pump I discovered it was a flex pump. Ever since then the truck runs fine until it is fully warmed up then if you dog on it it falls on its face and won't go (but never dies). If you baby it it will accelerate on but it is leaning out and throwing lean/missfire codes. The scan tool shows fuel falling out, inj pw increasing and ltft increasing so I konw its fuel related.

What exactly is the difference between a flex pump and a regular pump? By looking at them they looked exactly identical, that is why I went on and put it in. We don't run flex down here anyway just plain old gas...

I have never heard of a pump do this, as I said when cold runs like a top under any condition but after an hour or so, runs great under no load but if you dog it it will fall out???

 

Flex pumps have more output to compensate for that E85 crap? Maybe the fuel lines/rails/injectors are a different size and the regular pump can't hold the pressure up enough. Did you change the fuel filter, too?