Clamp is one thing but a "forgotten" element is the shim. I use to get this issue with the jetstream, I change the shim because the old one had signs of wear (aluminium deposition, galling marks etc...) which were creating a point interference once clamps rather than line/surface. New shim for a ~£2 fixed that... even with a thin anodized seatpost.
I also went for a cane creek seatpost whish is made of an Ali bar/tube milled on the OD and retained small milled grooves providing grip. This does not move even on fast gravel rides...