I had a seagull yak that had this same problem in a bad way. I did a whole lot of research about it and this is what i came up with. Check the easy things first. Do both elevator halves match and move together. Could one half of the elevator be flexing under load? Check lateral balance. Seal all hinge gaps. Reduce elevator throw to only as much as you need. If you need 40 degrees for 3d then so be it. Lastly drop any un-needed weight. A heavy airplane will snap out because of the higher stall speed. The lighter the batter. My plane was snapping dangerously and I fiixed it by perfectly matching the elevator servos. I switched to CF landing gear and wing tube. Smaller battery. Lighter muffler. All in all I shaved 3/4 pound off the airframe. Also make sure you seal all hinge gaps. This is a major part of it. After I did all of these things the problem was gone. i did them incrementally and the problem got better and better. I believe it was a combination of all the things mentioned. One last thing, check the horiz. stab and wings for any warping. Get them as close to matching as you possibly can.