how can i stop my traxxas T-maxx nitro r/c car from overheating?

use a heat sink to absorb some of the heat. you might also get a bigger motor or a bigger pinion gear.
References :

now over heating you mean it runs too hot and stalls? if thats the case richen the mixture a little there is a little gap you need to hit to give you peak performance but wont over heat your motor even if its running hot and hasnt stalled slightly richen the high speed needle on the motor and that should take care of your problem
References :