If it was shooting accurately but suddenly started throwing them everywhere, then all that leading in the barrel happened when you were shooting and has now caused poor accuracy.
Get some copper Choirboy (make sure it is 100% copper!) or some 0000 steel wool and wrap around a brush, it will strip the leading out nicely.
Idaholewis has said that he finds an 1/8" lubed felt wad between powder and conical drastically reduces leading. He believes it protects the base of the conical from being damaged/melted, so it both reduces leading and increases accuracy. He also recently did some shooting with maxi-balls, he shot 3 with both grooves lubed and 3 with just the lower groove lubed. The ones with just the lower groove lubed shot night and day better. He believes that giant upper groove doesn't shed all of the lube and results in it flying down range while still having a glob of lube on one side. That's gonna make for some really poor concentricity.
I've had much better accuracy with 2f when using conicals in my .54 Renegade, although it shoots 3f fine with patched round ball (PRB). Mine likes the Hornady Great Plains bullets so I don't use a felt wad with them. It does result in my seeing some leading in the bore. I much prefer to just shoot PRB's in mine.
I believe if you get all of that leading out of the barrel and then try some maxi-balls with just the lower groove lubed and an 1/8" lubed felt wad over the powder, along with trying some 2f, that you'll find some pretty good accuracy.
For a PRB load, TC's tend to like a thicker patch that is barely lubed. I'd start testing for an accurate load with a .490 RB and try both .015" cotton patch and an .018" pillow ticking patch. Lightly lube them with the lube of your choice, by lightly lubed I mean barely enough on them to rub in clear out to the edges.