For diagnostic purposes, move the clevis out to the farthest hole on the control horn. There is a possibility that the setup ie. servo arm / and servo that you are using, coupled with the inward hole on the control horn, could be the culpret. You are fine with a Z bend and a straight shot with a clevis (like you have). I know it isn't what ball link fans want to hear, but ball links can cause flutter by twisting the control horn. Many flyers only use them on throttle because of this. If you feel comfortable to fly it after the linkage adjustment, let me know and we'll go from there. If not, I suggest a larger servo. I'm almost done framing a GSE540 now.