? Reason for 2" high / low groups 18 yards with hot-rod

What would be some reasons for 2″ high/low groups with 18″ talon with o-ring mod, hammer slap mod, heavy hammer, polished and ported valve?
I know that it is inconsistent velocity that is cousing the high and low impact. The top hat is set very low, all internal parts are polished to a mirror finish. I make sure that the breech is all the way back, the 90 duromiter o ring takes some slight pressure to “pop” over and get onto the end of the barrel.
What are some of the most likely causes to look into. I will set up the chrony tonight but about how many fps should I be dropping between shots? I have the power setting set to 10 most of the time and on a 3000psi charge I will push a .22 21g kodiak at about 840 fps.
Thanks for the help!