That may be exactly the problem. Digital glitching due to over-powering the servo. Digital servos tend to twitch quite a bit with a fully charged battery when they are new. Mine do it all the time but there is no problem once the battery runs down a bit or the servos wear in. That's why I told him to run his battery pack down a bit. The regulator is providing full current to the digital servo causing it to glitch. Plug in a slightly run down 6 volt pack and it should go away. IF that is the case then it should be perfectly safe to fly the plane. I run my Rx off of a 6 volt NiMH pack without a regulator. I get a bit of twitching when I first switch on in the morning but I know what it is and it soon goes away. If you have the pull-pull cable system tight it will make it worse. BTW If you don't already, you should have one cable mounted on the bottom side of the servo arm and the other on top so they never touch.