Are you definately, positively sure its the scope? Why don't you try shooting a group at 50 yds with the open sights. It its shooting tight then I would suspect the scope. But if its erratic with the open sights too then its either the gun or the load. If you determine its the load, then try some Pyrodex RS (loose) or some good old FFg.
When I had my Optima, it shot great with the Pyrodex pellets and the 245 PB.