Welcome to Pennsylvania, and welcome to the unique and wonderful world of shooting flintlocks. Pennsylvania is the only State in the Country to provide a special deer season for flintlocks, and the vast majority of those still shooting flintlocks, hope that it remains just as it is.
I would suggest that you back off a bit with your powder charges, especially since you're using FFFg. At the range, start with 50 grain charges and work your way up, keeping the patching and lube consistent. After 3 to 5 shots with a particular powder charge, swab the barrel,(and this doesn't mean to clean it thoroughly using several patches). I usually use a wet patch and a couple of dry patches between shot groups. Now, step your charge up 5 grains, or so until a group indicates that the barrel is happy with that particular powder, patch, and ball combination.
Rubbing alcohol is an important product for the flintlock shooter. A patch with alcohol is used often on the flint and the frizzen to insure a good shower of sparks from quality black English flints and French amber flints.
Pick the touch hole between every shot, and don't overdue the amount of priming powder. Keep it below the level of the touch hole.
Your 50 to 75 yard range is pretty well dead on. Beyond that limit, your blade front sight will blank out your target quite a bit. You might consider squaring the bottom of your rear sight, if it isn't already.
Each flintlock has its own personality, and will cause consternation at times, but you must be patient because you and your rifle will eventually come to terms, and from then on it will be a long and enjoyable partnership.
Have a great year, and again, welcome to Pennsylvania!