its alsways a good idea to tighten the little "prong" against hte HSN for extra security, sounds to me like it may be running lean.

you should try tuning it so that its running rich on teh ground (but still gives enough power to fly reliably) because high up in the air it will run leaner, and thats probably where your problems are starting.

tune about 3 clicks rich on the ground and see how you go .

the fact that it takes 5 minutes to start acting up doesnt sound like a glow plug or fuel system fault... i could be wrong though