If I were working on it, I’d sit by the engine with an inductive timing light until the engine dropped rpm, then attach the timing light to one plug wire after another. I’m guessing that a plug is dropping out of the equation: The rpm drop is too low for it to be fuel, I think. However, ethanol seems to be taking a toll on fuel-related components. Hook up a fuel-pressure gauge in-line. If the rpm drops and the fuel pressure has a corresponding drop, the fuel pump is probably getting ready to die.