I had a Cannondale road bike years ago, that I could never get the front fork tight enough on. If I tightened the headset up enough that I got rid of the rocking, then the headset was compressed too tightly and didn't rotate freely. But, any looser, and i'd get the rocking. After lots of screwing with it, i finally took it to my favorite LBS (who i actually bought the bike from) , and they agreed that something weird was going on with it, and it wasn't able to be correctly adjusted, and recommended i install a new headset, which is what I (they) ended up doing, and it was fine ever since.