I had a problem with misfiring under load and at speed. It was an ignition problem. Replaced the rotor, cap, and plug wires (had installed new plugs previously). Runs great now. I found that choking the engine slightly, when warm, helped alleviate the symptoms prior to the tune up. Sounds like you may have the same symptoms. (I also started with the idea that it was the carb because it was running rich based on the condition of the plugs.)
