Stripping pinion gears

[nastety92]

I have a mostly stock E-Maxx with stock motor. I've stripped the two pinion gears, they are RRP 18T gears, it also has a RRP slipper clutch. I've replaced the pinion gears and now I can see why they stripped. The slipper clutch wobbles when it goes around. It is tight and everything looks normal. What would cause something like this?

Thanks,
Nick