I had the same problems with saddle movement, I am a bit heavier at 188. The bike had a single bolt setback seatpost and I had problems with movement in every direction. Seatpost slipping, fore, aft and tilt movement.

Just a few days ago I installed a Thomson Elite Setback post with a Surly Constrictor clamp. I'll wait and see if this works.

I can live with some stuff, but not seat/seatpost movement. Drives me crazy and it's a pain to readjust if your picky about the saddle position.