You said you checked the ignition system . Did you pull the plugs and wires and check the wires for "hot spots " or carbon track ,it will look like chalky white spots where the spark has come threw the insulation amd make sure no moisture is in thr plug holes (water or oil ) . Also what plugs ,timing ,and gap are you running . This sounds like a ignition cap or wire issue as it does it under load . Also double check fuel pressure .