The best thing to do would be use the manual (if it didn't come with one, download it) and re-build the diff. I assume it has the standard ball diff in it. It should be smooth and tight enough so you can only just move the spur gear with your hands when you're holding the wheels.

Also a 5.0 motor is huge overkill for an F1!!! They normally race with 21.5 or 25.5 turn motors.