I'd say it was the pilot jet setting. When you open up the throttle you give it a whole load of gas and air, but if the ration is wrong it will cause it to stall from such low revs. i'd try richening it a bit and see if this helps. On one of my race bikes (two-stroke, but same principal) I had a similar problem where I could bump start it and pootle around the pits, but opening the throttle much caused it to stall - you had to s-l-o-w-l-y feather the throttle to get it revving. A richer pilot jet solved it.
You only need to adjust your air screw - worth a try!