Since the first group is also you best, I'd go down to 70-75 grains, just to see what happenes...

Also...I have seen guns that really needed more cleaning than you are doing...And one that I had to not only clean with 3 damp patches, but then oil the barrel and run a dry patch after the oiling...

One thing about muzzleloaders, you are handloading each shot, everyone thinks about being consistant in the loading process....But the barrel also has to be conditioned between each shot as well...