First guess would be inconsistently loaded ammo, or a weak recoil spring.
Mag springs could be a part of the problem, if the recoil spring isn't up to snuff, and the mag springs are very strong. All of this would be made more noticeable if the rounds you're firing are tepid.
Mag springs and recoil springs, from Wolff, are cheap. Get the recoil spring shown as "factory weight" -- don't go higher at this point.
Examine the chamber area of the barrel, too, to make sure it's bright and shiny and not gunked up or otherwise contributing to the problem.
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