gonna be hard to diagnose without seeing in person but here's a few things to consider:

1) Do a complete rebuild, inspect for worn/broken parts, inspect/clean bearings and apply fresh grease on all rubber seals in diffs/shocks and refill with fresh fluids
2) What is the IR of the battery? More info here: https://www.rcgroups.com/forums/show...er-is-the-best
3) Has the motor been overheated? If so then permanent magnet loss on rotor may have occurred, consider replacing motor, more info here: https://www.rcgroups.com/forums/show...nics-in-my-car
4) More troubleshooting here: https://www.rctech.net/forum/showpos...ostcount=12161