Interesting. Especially since you mentnioed that it used to hold tighter groups. Is your first shot of each range session consistent? From a cold, clean barrell, where does it hit. Now completely clean the gun including the breech plug, go through the same prep & load routine (pressure on ramrod, etc...) you would when pulling it out of the safe. Fire again without adjusting the scope. How does that hit in relation to the first shot? I would do this 3-5 times and see if you get a group.
If no group, I would pull the scope, drop down to 50 yards and repeat with open sights.
If still no group. Then it is probably either barrel or load. I would take a look at the JB Bore paste others have mentnioed and start playing with other loads (sabots, bullets, weights, powder, etc...).
Keep us informed.