C'mon guy's let's put our thinking caps on. Do we really think heat is causing the problem? A steel rod three feet long gets longer by .040" with a change in temp of 200 degres F. , very small indeed. also if heat lengthens the pushrod the same heat is there to lengthen the fuselage, no change there. and fiberglass isn't mostly plastic it is mostly glass and glass has about the same coefficient of thermal expansion as steel.

Expansion is not the problem there, I would suspect the heat may be changing component values in the servo amp.