I have always thought that truck would make a fun toy,
Have you run 3 tanks of fuel and broken in the engine correctly? I have heard it has some duribility problems where your problem is, I would look over the drive train carefully for any stripped gears or backed off screws, any screw you remove put thread lock on it before reassembly,Gas trucks do require some work, try some nitro cleaner to help with the mess, I race a RC10GT myself and is a little work to keep it up but boy is it worth it!!!
check out the manufacture web site for help, contact your local hobby shop or find local racers who should be more than happy to help you......check out our club website