It may seem obvious, but make sure you are not resting the gun on the barrel. This will definitely affect your grouping (drove me nuts BTW).

If you have a modified pin-type probe, it is possible that as the bolt wears, it puts more sideways pressure on the pellet as you push it in. This can deform the skirt of thin-skirted pellets like the AA-Field and the pellet then fails to grip the rifling. Thick skirted pellets do better in this situation, like FTT or Bisley LRG.

Finally, if the gun has gone over-power, you will get shot-gun like groupings. I think this is because of the relatively fast twist-rate of these barrels, which were originally designed for lower power. I suggest you take off the silencer and measure the power with a chronograph.