I have a goldwing mx 2 30 ccer and I don't have this problem
it's hard to believe if AJ flew this plane she's act like this.
My elev joiner wire was much stiffer but I'm now using solar d 770's
while getting a 50 ccer may change the issue I think a new plane in general will.
There is somthing funky going on as this is not typical of a 3dhs plane.
Just what does your plane weigh.
And of course I'm sure you checked your lateral balance.
Of course if you have one servo moving faster or is weaker then it could follow the problem.
I know of several people using a single elevator servo but you need a really stiff joiner.
Also make sure your not cross coupling when you pull on the stick.
Good luck