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My first thought was your belt. 1.25" is really narrow. The wider the belt, the less likely it is to sag.
I've also found that even with a really good fitting and stiff belt, your pants and the location of the belt loops as opposed to the clips on your c-tac make a difference as well.
I'd try Chas' idea by going to an OWB setup for a bit and see if it helps. If not, you may just have to experiment with different belts and/or holster location.