Hollis, if the twist is 1:48, it seems like it's probably a compromise so as to handle either patched ball, or maxi balls. A .54 caliber should have a slower twist (e.g. 1:70) for patched ball, and a faster twist (e.g. 1:32) for maxi balls.
Check your patches. Do they exhibit any burn-through, or other anomalies? Next, check the bore. Is there any rust?
If you haven't tried a "natural" lube, you may want to try this. 50% beeswax, 40% Crisco, 10% Canola oil. I use this with .020 cut pillow ticking, and have a good combination for my Lyman GPR .54, and my various .50's.
You didn't mention the powder, but you may want to try Swiss or Elephant, if you're now using GOEX. If not, switch to GOEX. My .50's, for the most part, like GOEX fffg, although they shoot OK with ffg. My .50 caliber Jonathon Browning and my Lyman .54 GPR both like GOEX ffg.
Are you cleaning between each shot? If so, try shooting four or five shots before running a single "spit" patch and a dry patch down the bore.
Also, since you didn't mention, I'm assuming you are shooting a caplock. If so, try standard CCI caps (not magnum). If you haven't done so, try putting a new nipple on. It's possible that the old one is burned out. That will certainly affect accuracy. The red ones, by Knight are great.
I can certainly commiserate with you. I spent the day at the range trying to figure out why my .50 cal Jonathon Browning Mountain Rifle wouldn't shoot tighter than about 8" at 50 yards. Until I changed to ffg, it was very frustrating.
I've tried all the foregoing at one time or the other. However, I believe the "problem" lies with the twist rate. Just one man's opinion, though.