I'm heading out to fly in about an hour so just for grins I turned on the plane and radio, the trims are still set at zero, the shop is at 95 degrees and it's humid. I took a shot of an ARF I assembled to sell that I installed the elevator and rudder aft to get the correct CG and used a 4-40 rod for the elevator and made up a pull/pull for the rudder. It never changes trim either.
I pulled out a piece of the red outer tube and slid in some 4-40 rod, a lot of room left inside the tube but it will work. Also going to change the CG with the added weight. Then I pulled out a piece of the Nyrod and slid a piece of 2-56 rod in it. Perfect fit and slides well.
When I was in Calif. and flying a plane with the nyrod the trim would maybe go out a click/beep or two but nothing worth changing out the controls for.
If it's more then a little trim change you have some other type of problem. Something is shifting?? A little is normal. What are you calling a lot??