Pat; if you set the small dial on the vernier adjustment to zero, and it changes, than the movement has nothing to do with the collet. As Charles and I have both mentioned, some Craftsman models just won't stay put in the housing.
I know I stripped mine right down, thoroughly cleaned all parts, and removed some minor oxidation. That blued steel inner housing is terrible for rust.
When I reassembled everything, played with the locking mech. as Chuck suggested, it seemed to be fixed...I just can't remember if there was some specific issue? I do remember scrubbing all the collected gunge out of the rack and pinion height adjuster; it was pretty crapped up. As Red says, "We're all in this together."