Sounds like the quill is loose. Have you inspected and lubed the expander wedge and bolt threads? Are you properly tightening the bolt on top of the quill? Could it be inserted too far?
Originally Posted by Shmef
Here's a page of Sheldon's that explains... http://www.sheldonbrown.com/handsup.html
Here's the text near the bottom that explains why inserting too far is a problem...
Too Low! Danger!
Note: Steerers are butted at the bottom, so the hole in the steerer is constant-diameter until near the bottom, then the walls taper inward in the butted section.
It is vitally important that the stem extender (or stem) is not inserted so far that the wedge is installed where the steerer is narrowing, or it could come loose unpredictably.
When this happens, only the edge/corner of the quill or wedge contacts the steerer, and it is trying to "grab" a slanted surface.
This is sometimes a problem on smaller frames if you try to insert the stem or a stem riser too far down into the steerer.