^^Dags I think you are missing the point. When the racket is mounted the placement of the outer arm and top and bottom standards should never change. Wen stringing the main and crosses you racket is going to be distorted there is nothing you can do about that. When the OP says the main arm is sliding I think he is saying the standard(s) is/are sliding together on the turntable. When that happens the racket will become shorter and wider and the side supports are all out of adjustment because the side supports are slinding down/up the outside of the racket as the standards move in because the side supports are connected to the standards. When adjusting the top and bottom supports they should just be touching but the tightening of the standard against the turntable should be tight enough they never slide.
I have the ability to drive people crazy, not sure if I was born with it or I learned it but I'm damn good at it.