Brocock/BRK Brocock Concept XR velocity SeeSaw

Gun hs run flawlessly for over a year with slight velocity variations in a 10 shot shot string right around 880 fps and extremely accurate out to 100yds now Im getting 880-884 for one or two shots then 860-865 for one or two shots continues like that no matter how many shots taken replaced breech O ring got quieter but no help on velocity now Im thinking regulator tried waiting longer between shots no help....has anyone here ever replaced or cleaned reg I feel capable but any thoughts suggestions or tricks you could share would be greatly appreciated on a side note not really showing up in accuracy but I know somethings changed cant live with that cause Ill question every flier I get here on out