I'm not familiar with you, your horse, saddle, or any disabilities of either.
But, speaking from a 60+ yo. How long has this been going on? Is it current? Or has this been a problem for some time?
Saddle fit for your horse and you is extreamly important, as you may well know.
The shifting in the saddle, may be caused from several things, least of all your age.
When all is well and good, the saddle fits you and the horse, the the only other thing that comes to mind is your balance.
Since you didn't mention your age, and assuming that's not the issue, do you feel comfortable otherwise in the saddle?
As age sets in, we tend to get smaller in hips, thus, that beautiful saddle we've had for years, doesn't fit us any more.
If your comfortable at a walk or a slow trot, the extra movement at a gallop, may present an issue.
Just this old man's opinion...