I don’t know if this will help, but out of my four p17 pistols I have a red dot mounted on the p17 with fiber optic sights, the adjustments on mine creep, so my groups would always change. The red dot solved my problem. The pistol that I have shoots 410 FPS with cheap Crosman domed pellets.
Try the Daisy wad.cutters. I get pretty much one ragged hole groups, with them, at 14 yards.
One other thing I just thought of....These thing are a pain in the butt to load pellets into. Could you be deforming the skirts on the pellets when you load them? I see where some are modifying the breech end of the "pump arm" so a pellet pen can be used.