Originally Posted by gordonzo
It's a good learning experience to try and diagnose the problem before throwing on some new parts. If you have already tried stonecutter's ideas without success then also try these.
If it happens 'on every second flight' and 'moving the tail rotor out' seems to help, it might be something as simple as the tail rotor slipping due to a crack or just wear. Try the spare rotor if you still have it. Or, very carefully put a smear of CA at the end of the motor shaft, let it dry, and press the rotor back on for a tighter fit.
You were absolutely correct. Turns out the tail rotor hub split out one side all the way through. Had to flex it to see the crack.
Not a very good picture, but hopefully you can see it. The rotor was able to spin on the shaft. It's a wonder it worked at all. When I put the new one on it was much tighter on the shaft. Flies great now. Thanks all of you for your help.