I evidently have one of the older models, because mine does indeed have a metal throttle arm
.
I would say definitely start with changing that. I have three 2-stroke glow planes with metal throttle arms and steel throttle cables attached with brass EZ-Connectors and have never had a glitch (all PPM). On the other hand, I have witnessed this type of set up cause serious glitching on other planes. There doesn't seem to be any rhyme or reason why sometimes it's OK and other times it ain't.