I would first check to ensure you have fesh mix fuel(no older then 26 days). How is throttle response when it is running? Next check the spark arrestor in the muffler its a little mesh piece, ensure it is not clogged. My last resort would be to check the coil, your situation sounds like a coil problem. This is a common occurence when a coil goes bad, the way it runs while its cold but not when it is heated up.
Thanks for the response! I was suprised to here there made in Sweeden still! I would of thought they would be made somewhere else