gr,

Bottom line is that many have been successfully flying the HV. I would suggest a search for any mods.
Things to do that are not costly:
Make sure you are using 25% nitro fuel with 20% oil, mostly castor.
Break-in the engine.
Try different props.
Try a high-performance glow plug...or the one from your Norvel.
Try varying the number of glow plug gaskets (shims).
Check the tank for proper fuel flow. Some Hyper Viper tanks were defective.
I would suggest the steel lines.

Good luck,
George