Boy this sure sounds like what happened to my CX2. Turns out in my case that it was a motor failure. Try holding on to your heli and power it up to full speed and watch the rotors. In my case it was the upper rotor that would suddenly decide to slow down to a crawl after about 30 seconds.

The effect in flight was pretty dramatic: the heli looked like it glitched out and forced me to throttle down immediately. Of course the bird dropped like a (spinning) stone.

I retried the test several times and decided that either I had too much friction due to mechanical failure, or upper rotor motor failure (which indeed could have been its bearings). I couldn't find any mechanical resistance to speak of, so I replaced both motors with X-treme 180's and the problem went away. My conclusion was that the upper rotor (right) failed or was failing.

By the way, my CX2 has less than 100 flights. I'd say approximately 25.

Hope this helps.

Mack