An hour of flight, over the Atlantic, my motor started to just shutter for a second, not skip, just shook, it happened about every minute for an hour. I was on the way to the Bahamas and past the point of no return, at 7500 ft. After I landed, I called my Mechanic, first checked for water in fuel, then plugs and wires. All looked normal but,,,,, I noticed a blown exhaust manifold gasket and exhaust blowing on one plug. My mechanic said it was causing a pre-ignition on that plug and when it fired, it was firing backwards on the motor causing a shutter rather than a skip. He told me not to lean the motor on my return trip, the cooler fuel should not fire prematurely. He was right, no shutter on the return trip. I really should check my exhaust gaskets closer. I know many of you know this already, but I just passed in on for the benefit of the one like me that did not know this.