First thing i would do is take cayugad's suggestion and use some JB bore cleaner at least 200 strokes.Then would take others like skeeter's and try different bullets but first I would swab between shots,what I do is take a patch and lick it on one side run it down the barrel in short jabs,if you don't the patch is most likely to hang up in the barrel and sometimes you have to take the breech plug out just to remove the patch and ram rod,then I turn the patch over and run it down the barrel and back out and then reload.Another thing is to let the barrel cool between shots,sabots don't seem to like hot barrels.
If none of this helps post your results and one of us may be able to help you farther.