[Herb Stoops]

John, I think that many people wouldn't use a miter gauge and a fence at the same time. As Charles and Harry said, I would use a square block against the fence, behind your work, as a pusher and try to shave a hair off at a time. I don't know your skill level, so I hope I'm not insulting you.

I agree, if the miter gauge is not square with the fence, it will do what you describe, i.e. If the fence is not parrellel with the miter slot then as the wood is pushed forward either pushed away from the fence or the leading edge will cut and the heel will be forced away from the cutter. By using a block that is at right angles to the fence and held against the fence like the drawing should solve your problem.
Herb