Have you been a old die hard bore butter user as well? That could be the problem. I had a T/C Renegade do the same thing to me, just like you're describing. To get the rifle back shoot;

I took boiling hot water, and dumped dish soap into it. I then gave that rifle a boiling water/soap bath. Be sure and wear gloves or the barrel will burn you. After that I dumped some solvent down the bore of that hot barrel and then using a bore brush took a strong solvent like Hoppies, or Birchwood Casey (at the time it was Hoppies #9) and scrubbed that bore with solvent and a brush. After that I ran solvent patches through it. This removed all the bore butter, all the lead build up, and anything else in there. After a lot of dry patching, I then oiled the bore up and it shot just like it always used to do.

There are a lot of solvents on the market that will remove lead from the bore. Read their labels and it will mention which ones. Also J-B Bore Paste will remove leading from the bore. After you get the barrel and bore back to normal and shoot it, let us know how it behaves. We must have one of the same kits....