I know you said you are using "new" fuel and a new plug but what you are describing is almost ALWAYS fixed by a new plug or fresh (dry), fuel. My guess is, is that you've got moisture in your fuel. The easy test is to borrow a tank of fuel from a buddy who's engine is running well.
Fuel will absorb a considerable amount of moisture without harming the running too much. Up to a point, you probably won't notice. Once it passes a certain threshold, the symptoms are the engine will start, idle, and run well but at random times simply shut off. It will then start right up again but a few moments after removing the glow starter, quit again. With yet a bit more moisture in the fuel the engine will run all day with the glow heater attached but shut off with it's removed.
Try some other fuel and another OS F. If that doesn't get it then start looking at tank, clunk, filter, pluming, mounting, valve, air leak, etc. problems.
Let us know what you find.
Cheers,
Bill
Last edited by wjvail; 04-29-2014 at 02:55 PM.