I put this on mine and never had to worry about it again.

http://www3.towerhobbies.com/cgi-bin...&I=LXUD52&P=ML

If you don't what to go that route. It sounds like you might have your gear mesh to tight or too loose. You might want to also take your spur gear off and check things out. Look for anything that might not belong, Check your pegs etc. When you install it back tighten it up tight and then maybe back off a couple turns, But keep it fairly tight. With the mesh you should be able to feed a piece of paper between both gears. They should also spin very freely with the paper out. Hope this helps