Breaking up under load is most always a secondary ignition (spark plug wires and spark plugs) problem.. there is a very small chance it's your CDI but I kinda doubt it. I did see a Speedmaster once that the CDI was bad causing it to break up in the higher RPM ranges... but not under load.
Change the plugs.. even though they look fine, it's happened to me twice now and it was the plugs both times. A lot of coils and wires have gotten replaced but it's my experience that is not usually the fix.
