The main reason to use a felt or lubed wad under the bullet is to help protect the base of the all-lead conical. Black powder burns at about 800-825 degrees, while pure lead melts at 620. The base can actually be melted/vaporized a little, and deposit lead on your barrel during the shot. I have never found them to affect my accuracy very much.
The power belt base will essentially do the same thing, and the properly fitted and expanded flare will scrape the residue off the inside of the barrel when loading, and firing a fouling load may not make much difference either.
Sounds like your gun doesn't like the bullet sizing , more than anything, and you would be better off with a REAL (rifling engraved at loading) bullet, or a properly paper-patched one to fit your gun. Trying for extreme accuracy out of muzzleloaders is a trial and error process. Good luck!!