it could also be your brakes.
i know that on my bike, the headset/stem is all adjusted just fine, and i still get that little knocking and slippage if i pull the brakes and rock the bike back and forth. but if i try to move the headset by bracing the bike against a wall and rocking it, i get nothing.
if you look at your brakes, try to see if there is a slight (VERY slight) amount of play between the actual brakes and the posts they're screwed onto. i'm not talking about the pads...play between the mounting posts and the 'calipers' themselves. there really isn't anything you can do to fix this...maybe add a washer between the bolt and the brake arm...i haven't done anything about it, since i don't ride that bike very often...i suppose it could eventually rip the brake studs off the fork, but i doubt it.
if it turns out to be the only in the fork, your cartridges might be starting going bad...if your fork doesn't use cartridges, then i don't know what to say...