I've had trouble with my older WoundUp carbon fork and steer tube. I have to tighten the CK headset quite tightly once in a while. I think what's happened is that the carbon steer tube has become a tad smaller with time and wear and is now under spec. The stem, tightened to spec, will slip on it after a while, particularly after fast and bumpy descents. I've tried a couple different stems on it. Stems with less bearing surface seem to do best. I'm not that anxious to buy a new fork: kind of spendy. The LBS has looked at it - no solution.