According to the Hodgdon site, with similar components (using everything you are, but with a WAA12 wad), 18.4 gr of Clays gives 1200 fps at 11,300 psi. Likely your load is over revved. I prefer to keep pressures over 9,000 psi, but barely over 10,000 psi. My HCP load with Unique generates around 9000 psi at 1250 fps.
17.3 gr of Clays should give you about 1145 fps at 9,700 psi. Why not back off the powder a little, and see what happens? This load cannot be performing at its best, and is likely above maximum recommended pressures. You could easily be pushing 12,000 psi.
Also, when you changed the stock, did you verify POI using the Winston method? A little difference in the stock can make a big difference at the target.
Neil's method is the Gold Standard in verifying where your gun shoots. :^)