When the engines tune is a little lean pointing the nose in the air will exacerbate the problem and cause the engine to surge or die completely. Next time you run the engine on the ground run it up to full throttle, pick the palne up and point the nose in the air and wait. Listen to the engine If it starts to surge or tries to die it is to lean. Also I would reccomend a smaller prop for the Magnum. That is alot of prop for a .70 and it may be overloading the engine in combination to a lean mixture. Try the APC 13x4W.