It sounds like the right side of the rear axle is slipping forward. The only way that I know of to stop this is to make sure that you have the right chain stay lug clamped between two strong serrated washers or nuts. In some cases it is possible to tighten down the outside nut and think that it is grabbing the rear lug when what it is actually doing is compressing against spacers and such that are also on the axle.

Also if these are fixed gear bikes or single speeds with no slack in the chain, then an out of round front chain ring may be pulling the wheel out of alignment.

I am just guessing but maybe this will help some.