[Chrislong]

Those spring washers ought to last donkeys. I know with the Losi diff, the best one I had was one built with the similar spring washers and was a diff bought 2nd hand - and id been using it 4 years on and off.

If something is gripping on the last thrust washer, or it is a little oversize, then that could turn the screw and loosen it.

Try rebuilding it - but when refitting, put it in the opposite direction, as if the forward motion is a factor that it current drives, then that direction becomes the braking force which is far less.