try using a shim, your bars might be ovalised
I think he's saying its turning not the bars going forward or backward!!
Take your quill out and give it a good clean down the forks. Maybe there is a load of loose dirt or rust down there that the quill can't tighten up on.
Also check the quill. Some quill ends were smooth and some had ridges so it could bite the inside of the forks. If its smooth then it wont help grip the stem.
Have you got another stem to try in it!! Any stem will do, just to check that its the stem at fault and not your forks!!!
Also if someone has over tightened a stem in those forks then it might leave an impression of the old quill inside the forks. When you try tightening your quill it might be making minimal contact inside the forks so that there is some play left and right but the quill cannot be pulled out.
Thats it, I'm all out of suggestions!!