How long have the spark plug wires been on the engine? Modern non-metallic "wires" deteriorate over time and develop very high resistance that can cause an intermittent miss under load. Try checking the wires with an ohmmeter. There should be no more than 1,000 ohms per inch- - - -for instance, a 12" wire shouldn't have more than 12,000 ohms. A wire bad enough to cause a miss under load will have much more. Good luck!
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!