A fan shouldn't be needed but can be a good thing. Keep in mind that you can get fans for both the motor and another one for the ESC. The clip on's for the motors are pretty cheap, like $10 or so. Cheap investment. Keep the same gearing and run a fan on the motor.
Just an opinion though.
And like Verndog said, try those adjustments. It sounds like you have more friction type heat with over reving the motor if it's that low of traction.