This is driving me nuts. Thompson elite seatpost (aluminum, 31.8 x 300mm) keeps slipping down into the seat tube of my Litespeed Siena (titanium). The seat tube is butted internally, which means the internal diameter increases about 4-5 inches from the top; below that butt, there's no contact between the post and the inside of the tube. I've searched and searched for any reports of slipping seat posts in Litespeed Sienas or other internally-butted frames, and found no other reports of slippage or any discussion of butted seat tubes causing seatpost issues.

I've tried using ordinary grease, titanium-specific anti-sieze compound, and carbon paste. None of them make a difference. I've tried 2 different seat post collars and I've varied the torque on the collar bolt. No matter what I do, it's the same: the post slips about 1 mm over 10-20 miles. It's exactly the same in hot and cold weather - differential heat expansion of aluminum and titanium doesn't seem to have any effect. I've used this same seatpost in a titanium frame before with no problems.

Any suggestions? Thanks in advance.