Try some Breakfree, G-96, the Mobile One formula or other good AR Lube on the spring and see if it stops. If the spring is bone dry it can cause this problem. It did on the Military Rifles. As far as changing a buffer if you do I would only go to an H or H-1 Buffer they are the same just marked differently. You should never need a heavier buffer in a semi-auto AR unless the gun has entirely the wrong Gas Port in it, which I doubt yours does being a Stag. Guns that are that over gassed that much normally display other issues like feed and extraction problems. Stag normally makes excellent rifles. Lube the Spring first it should take care of the problem.