acceleration problem w/ turbo silverado

[GZUS1985]

i have a rear mount turbo silverado 4.8. the problem i'm having is when i press the peddle down to accelerate. It almost seems like too much fuel is going to the engine and it slows down a lot. but if i have the car in park and accelerate the engine will run fine and the turbo spools. when i drive on the highway it'll run kind of smooth. but still trouble when i accelerate i have trouble w/ the engine wanting to do more, and the turbo doesn't want to build the pressure. does anyone know what could be causing this problem?