I would lean toward running on one cylinder. When it starts running poorly then shut it off and remove the spark plugs, and reinstall them in the boot and hold them to a bare spot of metal on the engine and have somebody crank the engine and watch for spark. Or get two of those inline blink type spark testers, and install them on each cylinder when cold and then when it starts acting up then see which one is not firing. Could be another issue like valve clearances but loss of spark on one cylinder would be my first guess.