what kind of push rods do you use ?
I think your push rods might be flexing ,when the speed pushes the elevator back
Your push rods must be attached to the inside of the fuse ,and to each other , so they --cannot flex --in there middle , when in flight
The increase thrust of a gust of wind ,could be enough to over power the rods ability to stay ,straight ,from the servo to the elevator horn
Always check the elevator horn ,for wear -loose
Use Ca glue to make the horns base ,a hard place