Loosen up the stem enough so that the fork can move freely. Now tighten the screw thats on top of the headset just a bit. Check to see if its better. If its still loose repeat again. Dont keep torquen the screw in untill its tight because its not supposed to get tight. Just give it a little turn at a time. Heres a link video:
Are you sure that there's the minimum 3-5mm space between the top of the steerer tube and the top cap so it has room to tighten down? Did you pull the bearings to see if maybe they're toast? Who installed the headset and how old is it? Best thing I can tell you is to check Park Tools website on installing/checking a headset and whatever else you might need help with.
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