I just put a DCX 500 on my Precident and experienced the exact throttle problem, I went into program and it was set on custom, I tried all the other throttle curves and selected the smoothest, I think it was the inverted curve, cannot remember. It still has a little pedal travel and if you do not give it enough throttle it is a little jerky but way smoother than before I reprogrammed it. It is really smooth once I have gotten used to it.
Download software at Alltrax. My cable was a radioshack DB9 on one end and USB on the other. Plug into controller port and computer, turn key on and then open Alltrax program and it should recognize the controller.
http://www.alltraxinc.com/Products_ControllerPro.html