initial thought is check the gearing. when electrics get hot and/or get hot fast its usually do to load. having the car geared to high will do that.
driving conditions can cause excess heat, lots of hard accelerating and breaking. or if its just really hot outside it'll cause extra heat.
extra driveline friction can cause extra heat, although in a pan car that's not likely. but still, check the gear mesh and make sure it's not to tight.
when was the last time you cleaned/sprayed out the motor? iv had situations where a really dirty motor was causing inefficiency and extra heat (as well as poor performance)
this one my sound dumb but iv done it, make sure you're not running the ESC in reverse.