Originally Posted by TryHard

Check your internal diff mesh. Possible that inside the diff one of the crown (big) gears isn't meshed right. Pull it apart and check that the teeth are all moving correctly. I'm not sure on the new tamiya gears, but have a close look at make sure you've not damaged the teeth. I've had this before and had to take the case apart and realign... teaches you quickly to make sure that the teeth are meshed properly before screwing the case down.

HiH
Ed