Loose dirt=going fast nowhere. Burning energy and power spinning tires. Yes it has to do with running your car on loose dirt. I would consider lowering your gearing a bit to a 15. Cheapest way to keep it cool since your bashing is to run it for a bit and let it cool or constantly monitor your temps. I can suggest to run with zero timing on the motor. Is a 4 pole motor and for what I understand 4 poles don't like timing advance. That's one reason most 4 pole motors don't offer end bell timing. But main thing is... easy on the throttle on loose dirt... Lots of wheel spin looks cool but contributes to burning motors and esc's.