If you are experiencing this on a foamie, check your hinge tape. Did you use the fiber reinforced, or thick clear tape? Some of the brands tend to not center well. You get it all trimmed, and then you give up or down, and it wont go back to exact center. It will always favor the side you just gave. Example- You trim it for neutral and then give full up elevator. The servo will then go back to center and the control surface will stick up just slightly. I do mean SLIGHTLY.
Try 3m "scotch" tape. The really clear stuff with no fiberglass.