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Sounds to me like an ignition problem. Under load higher voltage is required to jump the spark plug gap. Several things can cause the symptoms you are describing. Faulty spark plugs and or wires, defective coil, defective condenser, or even points that need to be cleaned or replaced. From your description, I would start with checking that the plugs are not fouled. The higher the voltage that is required to jump the gap, the more it is going to "seek" a shortcut to ground. A carbon trail on a plug or a faulty wire can do that. Points and condenser can also do that, but are not so sensitive to the load of the engine. A coil can also do that, but usually don't show symptoms till the coil heats up. Start with plugs and wires.