Ok a few things to try,

Rebuild the motor, fresh cut, new brushes
Try different motor - caps can be damaged, esp if buzzed on a 6 cell pack.
change crystals, can be damaged esp in offroad.
pull case off receiver and brush clean, dirt and rocks can sometimes affect the receiver.
make sure all radio components are properly insulated, and wires esp aerial do not touch the graphite material.
Add extra layers of servo tape to receiver and speedie to elevate from chassis.

Let us know a few more details, as it may be able to assist.