[Krawlin]

This happened with my Mugen MBX5T and it's nitro, not a hell of alot of power being thrown at it. I would say that due to the nature of helical cut gears (wanting to push away from each other), the bulkhead is flexing and letting the pinion deflect from the ring gear. I would agree with what has been previously been said about having too light of a center diff oil though. Running a heavier oil will slow down the front diff and the ring/pinion wont be trying to deflect from each other as much. If it still happens after changing the center diff oil I would try replacing the bulkhead with a brand new one.