When the engine suddenly quits, it is referred to as a flameout by most RC heli pilots over here in my part of the world. Are we talking about the same thing? If so, it can be caused by engine air leaks, lean mixture settings, a bad glowplug (which can be caused be either of the former), poor engine cooling, and bad fuel. If one of these factors makes the engine overheat, it can tighten up and/or lose proper combustion timing and quit.