There could be a ton of reasons for flameouts. Like others posted you should first look for a hotter plug, the RB#5 or the C5TGC Nova plug. The F11 makes a lot of power so even tho it feels fast it is probably still rich and the #6 plug that the engine comes with does not burn hot enough to keep an engine running at a rich setting. If the OS plug is all you can get they will work but try to always use the OS once you make that switch. I would try the Nova C5TGC first honestly.

How much fuel have you ran thru the engine? When does it flame out? After idling for x amount of time? Only when you pull full throttle? After pit stops?