I was not able to fix it. Eventually Look agreed to replace it under warranty. I took a very close look at the upper edge of the head tube and there was what appeared to be a hairline crack. It was maybe 5mm long. As soon as I mentioned that to Look the warranty was accepted

i got the replacement frame last fall. Within 3000kms on the new frame my headset came loose again. I’ve tightened it once but I still get a small amount of play sometimes. Yes, you heard me right: sometimes. I have no idea how or why but I’m not going to tighten again until I have more play. I absolutely love this frame but I’m concerned that the design may have put too much strain on what seems to be a flimsy headset assembly.