acceleration lag???

[chris1221]

for starters '03 6.0 f250. had numerous problems such as hard starts, bad acceleration, and just all around running like crap. i've fixed and replaced many things, most recently the accelerator position sensor. aside from the occasional long cranking before it'll start most of the problems have been resolved... except one. at any temp- cold from start or at full operating temp.- if i mash the pedal, the motor will rev to around 3500rpm, stick and wont shift. however, if i drive it easy and gradually accelerate it runs and shifts just like it should. my guess is a sensor of some sort. my question is which one? thanks in advance for the advice.