If your engine overheats it will happen in the air. All the times I have had it happen, It has been with different engines and came directly from setting a bit to lean. About 2 minutes into flight, I would lose pwere slowly to apoint I am full throttle on the tx and going nowhere and land with the engine running at idle speed and stick still full forward.

I've had it happen enough times to clue into it before anythng really bad happens.