Maybe the bearing's have worn out to the point were there is enough slop for the clip to fall out? Just a guess but it's hard to diagnose a problem without actually looking at it.If the gear's show wear on the inner edge or the outer it mean's they are out of alignment. They should wear evenly, if not you might have to use shim's.Make sure you use locktite on the pinion bolt, I've seen them back out.