Visually inspect cylinder clearance at the chamber position where it binds to ensure that cylinder isn't making contact with something it's not supposed to -- such as the forcing cone or top strap. (Was your revolver ever dropped?) (Could the cylinder be out of alignment?)
Could be some gunk build-up inside the works (but not all that likely). Check the hammer as it follows through its rearward and forward travel and see if it might be rubbing/slightly binding against the frame.