The valve will give you problems if the clearence disappers when hot. It would normally hold the valve open and compression would dissaper. Have you tried running a softer plug. I use an NGK D6ES in mine.
The exhaust heat shielding is important, not too difficult to bodge something on for a trial.
Also if you wind on the throttle too fast it will cough and run rich, open smoother.
Final thought is a resistor plug to blame as I use one without and electronic XL ignition.