I have had this problem for some time now too! All of a sudden stick goes out of calibration. I have a
Saitek X52. It was recommended that I get a powered hub, did so and it didn't fix the problem. I still go out of cal at least once per session of flying. Even off line.
Don't ever remember having this number of cal problems with my old Thrustmaster.
I am wondering if maybe the T1 to T6 switches may have something to do with loss of cal? Sometimes my tubby tummy hits them.

I haven't tried this idea scientifically yet. (ie. hitting these switches on purpose while flying.)