I bought mine from new and I had a sudden drop in power. I sent it back and they adjusted the grub screw above the reg and that seemed to have sorted it and I'm sitting with 10fps variation over the chrono.
Have you recently cleaned and lubed the internals ? I made the mistake of putting grease on my hammer and hammer spring and as the gun got colder the pressure dropped as the viscosity increased. I know a stupid thing to do but happy to share my mistakes.
It would be great to see your dipped hammerli. I 've attached a pic of mine