Either interference or binding on the throttle servo, check the throttle linkages really well, especially the 90 degree elbow the server attaches to on the side of the chassis in front of the tank. Also check the slider movement at the carb without the servo attached to it.
If that's all good check for metal to metal, that usually makes the servo's twitchy not lock in place.
When my stock steering servo went it lost the ability to center itself when the drive gear stripped, so it would intermittently stop working too! untill it was completely stripped, then it just clicked and tried to move.