hrm... if the difference is installing the pinion... does it only do it when the motor is running, or does it do it all of the time?

If it only does it when the motor is running, likely something in the motor or drive system (if you have one) needs lubed up, or your pinion could be slipping.

Also check your antenna and see how it is routed. You could try laying it to the side of the plane just straight out and see if it still has the same troubles.