My e Revo Mamba Monster will not keep a Pinion on the shaft. It hasn't done this ever until about 2 weeks ago. Stripped out my spur gear, got another, stripped, got another, stripped. I red loctited the set screw threads and screwed it in the gear. I ran on a 3/4 stripped spur gear today, and it still slipped on the shaft. I don't know what to do. What's causing it to be pushed forward and stripping my spurs?