My advice in the short term is to put carbon fiber grip paste on the cones to try to stop it from slipping-- because it's gonna try to slip. My longer term advice is to buy a different seatpost. I dealt with a very similar Bontrager post awhile back, and it was relegated to the parts bin rather quickly. I recommend the FSA SL-K ITC post-- it uses a three bolt design, so you can adjust fore-to-aft and tilt independently. Also fairly light, and pretty cheap-- I've seen 'em on eBay as low as $30.
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