Mine has a lot of play too, its just the design. The window rubbers are what guide it up.
But if yours is pinching halfway up, it sounds like the window isn't fitted straight. There's a post buried in here about the exact distance the window needs to be mounted from the rubbers to stop wind noise, water leakage and wobbly going up.
Get some copper slip on the moving rails anyway - with all the water pouring into MK2 doors, that mechanism gets awfully dry.