i just had that happen to mine, i just got done replacing the inner shaft. i just use the plastic ones. they're only a few bucks at my hobby shop. also, it matters what way the collars go on. and theres also a washer that goes on the inner shaft that has to be facing the correct way as well. they have a little lip on one side of them that is designed to hold down the bearings they sit up against. but if the teeth are chewed up you should notice an increasingly noisy sound when you give it throttle. does this happen?

now, if i can just get an answer to my question...