How is the compression pressure? Burned valves could account for most or all of your symptoms. (I suggest you check the tappet clearances as part of the same investigation.)
Have you checked the free fuel flow rate out of the fuel tank? (It could be starving for fuel under load.)
Have you tried inserting an in-line spark tester when you run it under load? It could be coil break-down.
Is the air filter clean and correctly assembled? (Obstructed filter could make it rich under load - you could probably detect this from the exhaust smell.)
A governor problem does not seem to be a good fit for the problem, since it seems to be temperature sensitive.
At this stage I'd say the leaky valves are the best fit for your symptoms, but the other items should be checked anyway.
