it's probably due to having too much weight in the rear from the motor. not too much you can do other then ballast the front with lead. The r is somewhat known for being less then ideal on asphalt as is, so when you take even more weight out of the front, it gets tail happy due to the motor swinging the rear around.
also, you could try stiffer rear springs to make the snap more predictable, and lower the rear roll center.