If you don't use that particular belt for anything but as a carry belt, you might consider mounting some adhesive-backed velcro to the front of the belt, and the back of the holster. It wont look pretty when the holster is off the belt, but if you wear it only for ccarry, then nobody will ever see the velcro anyway...
Or maybe you could mount some adhesive-backed non-slip shelf-liner material on JUST the back of the holster. This will help keep it from moving to much, bot won't be as secure as velcro.
It sounds like the reason it might be moving around is because your belt is too narrow for the holster loops. If you get a bigger belt, it won't move around as much. Also, thicker belts will also help. A thin belt will sag, and lets the loops slide around too much. The thicker and wider a "carry belt", the better!
Good luck with this.