Do you have a temperature gun to check the motor temps? You're best bet it to buy one and monitor the motor temps for each adjustment you make. Make sure there is a "tick" of play between the spur and pinion gear. If you have the two gears mashed together, that can cause unwanted friction which will generate motor heat.
Also, drive the car for 2 mins then temp and then adjust the gear ratio and motors timing.
Are you running any advanced timing from your Reventon Pro?
I am running the same setup with no problems at all.