RE: R127DF and Hitec Digitals, save your plane and read
Watching for a voltage drop with a scope at the servo will give us an idea of what the servo is seeing at peak load. To some point as the voltage starts to drop the current required to provide the needed torque goes up. Now the second thing that can start to happen is you can get a voltage drop at the battery (more prevalent with NiMh batteries because of the higher internal resistance).
Next...another very interesting place to look at with a scope is the switch. Just because a switch is called heavy duty does not tell us the contact resistance and the current carrying capacity of the switch. A voltage drop at the switch for even 100 milliseconds can cause the discriminator or the decoder in the receiver to do a lot of strange things...glitching can be one. This can be very different from one receiver to another.
Another place you will see voltage drops is on the contacts of low cost extensions. There are some on the market that skip the gold plating to keep cost low. Using servos the require higher current with these just make the problem grow.
You will not see any of this with your voltage watch. Even if we could I know my eyes are not quick enough to see a 100 millisecond voltage drop on one. They are very good for showing the average condition of the battery.
Your problem may not be any of these things...just something to look at and think about.
If you lived closer I would invite you over to look at your system on my scope.
Good luck, I hope you can solve the problem.
Pete (HiTec service center)