I'd also look at the patches, you are asking a lot out of a .015 patch with 120 grs of powder behind it...What do they look like???
Also...Are these store bought pre lubed patches??? They can go bad...
My .54 did this a few years back...The fix was to buy pillow ticking in bulk and lube when I was going to use it...
I've also see guns start this because of damage to the rifling at the end of the muzzle and actually cutting the patch at loading, then burning a hole...I've fixed a few by filing the rifling and correcting...