I don't recall the OP stating what size airplane, servo brand and model number or a picture of the setup showing us mechanical advantage. While most of what I read regarding the cause of flutter is correct, the most common cause on an airplane like the Sbach is improper setup. Things to check is whether the servo has the power for the job, the servo arm should be at least 1/4" shorter then the control horn and there should be no slop in the linkage. Being that the first few flights on a fresh RX pack went well clues me in a a little . With slightly higher voltage the setup had enough power to not flutter. Look to make sure the servo has enough power and you have good mechanical advantage. Pictures would be a big help.