My 2001 two-stroke Johnson 115 has an intermittent bang at over 3,800 RPM. It sounds and feels almost like you hit a log. It's been doing it for about 6 years and seems to be getting more consistent. It started The next season after having the lower unit replaced. At first it was very sharp sound like two pieces of metal being slapped together. It was very intermittent like once every couple of miles. Now it's a dull thud and can happen as much as every hundred yards. However if it were something in the lower unit I would think it would have blown apart by now. Maybe something electrical causing no ignition for a short second?