If you have enough stray signals from your ignition to find it's way down a cable, through a servo arm and past all the gears to get to the servos electronics you might want to check out some other ignition units. I've flown my planes with less than an inch gap between the reciever, servos and wires with CH units as a test. No problems!
The glitching you are experiencing should be duplicateable if you place the reciever or antena wire up near/on top of the ignition, heck wrap it around the ignition to see if it is sending stray RF interferance. If all servos start glitching then suspect a ignition problem.
Sullivan makes the best flex rod for throttle control. It has a small amount of CF and slides super smooth. It doesn't expand and shrink with temp changes.
Your fix will most likely be a replacement servo. Go to Towers site to read the specs for the servos you are using. Sometimes they put a note in the specs, (not for airplane use) it generaly means people are having gittering problems with a servo and they don't want to have to replace them. The note usualy disappears when a new version of the servo comes out.