have you changed the gearing? or the wheels? 250 is VERY hot. safe range for most brushless motors is under 180. some will last a good amount of time at 200-210. 250 can ruin the epoxy.

if you have not changed the gearing or tire size, check for any binding in the drive train. Something is causing the motor to work harder than it should have to.

A fan can bring down temps 10 degrees, maybe 20.