I could be totally wrong here but my garelli did that when i bought it and After i opened her up I notice there was 10 teeth missing on the driving gear. The gear would slam onto the other one when it hit the bare spot because the two gears were not lining up correctly. This may not be your case but I guess its possible you have a bad tooth on it someplace and every now and again its going to catch. Just trying to help but I am a begginer also. Good luck with it, I'm having a blast with mine.