I recently build my Tamiya Porsche 911 GT3 in Aug 2009 (TT01-E chasis)(2007). When I hold it up in the air and accelerate it works fine BUT when I put it back on the floor and accelerate, it moves really slowly???

Is it the ESC??? I have even taken the car apart and rebuilt it which was a WASTE OF TIME and have not been able to SUSS OUT THE PROBLEM....... Can somebody PLEASE HELP AND SOLVE THIS PROBLEM. Really want to get it up and running..............
HIGHLY APPRECIATED
THANKS IN ADVANCE