My SX is rock steady on GPS hold, with minimal wind(5-8 mph). Rock steady defined as a VERY tight 3ft circle.... If any movement or drift at all. I rarely get a 1ft variation in up or down altitude, but for the most part it stays put.
BUT... I have noticed when I drive to a new location and fly, as close as a 10 miles away, the GPS hold seems to"hunt" around a little more when in a hoover. Maybe drift to a 5-6ft circle in hold. So I would land and recalibrate the compass then fly again, and it would be rock solid again. I thought it was weird that just a small change like that would make such a difference. Strange I know. So now I recalibrate with every flight, and rarely have any problems.
Have you considered something like that?....
While this map doesn't show your location, this map may give you an idea why you have to recalibrate even if you only change locations by miles. http://www.geometrics.com/assets/images/usmag-poster.pdf
Jeff