David - sounds a lot like you have some kind of electronic interference issue. Are the two servos you've had trouble with the only ones you have had in that airframe? If yes, then I would most definitely go searching for an RF related issue. In my many years of doing these, I've found and heard of so many different servos that *do* work on gas engine throttles, that I tend to pretty much dismiss a servo issue initially. Look elsewhere, particularly if servo is anywhere physically near a kill switch, or ignition battery, or ignition, and if you are on 72Mhz, it could be practically anything metal-to-metal on the airframe ANYPLACE creating throttle blips like you describe.