I have the same issue-- completely different bike, but two different posts, three different clamps. Still slips about 1.5mm over 20 miles. So I went to what I would consider the nuclear option, a double-bolt clamp, like the kind used on unicycles. The clamp has two collars, one to grab the post, and one to grab the frame. No slipping, guaranteed. Not for the weight weenies, it's over twice the weight of the clamp it replaced. You could also try getting a clamp for a 27.2mm post (which is for a 31.8 frame) grind off the little ledge in the clamp (if there is one) and tighten that clamp onto your post above your primary seatpost clamp. The belt-and-suspenders technique.