Help: I have a Giant Defy Advanced carbon bike that has a godawful creaking sound coming from the seat post/down tube area. When I stand on the pedals to sprint it goes away, but when I'm on the seat and grinding away it woulds like I am walking across a 100 year old wooden floor. Skreeek, groonk, kruuuunch, skreek. Nobody wants me in their pace line. The bike shop recommended reassembling the seat post with carbon assembly paste. The stuff feels like pumice in Vaseline, and it hasn't helped. When the bike was assembled with just grease, the seat post dropped an eighth of an inch each time I hit a bad rut.
How can I get rind of the horrible creaking and still keep my seat height fixed while riding?