My stem keeps moving side to side even thought I tighten it I just got a new Demolition Stem.I tight it with all my force And It keeps slipping side to side Like it doesnt match with the fork lined up And whenever I check it its fine but after a while it is always not lined up with the fork..Please help me
Is it possible you tightened the pinch bolts before tightening the bolt that runs vertically and holds the top cap down?  If you do that, the entire assembly won't be fixed tightly on the vertical and will allow side-to-side play.  I did that once and the symptoms were exactly as you describe.  To check this, loosen the pinch bolts and the bolt in the top cap.  Now tighten the top cap bolt first, then the pinch bolts.
Be very carefully while tightening the top cap. If tightened to much, it can overload and ruin the headset bearings.

Here's an image from Sheldon's site showing the bolts I mean: