I hope you got this fixed... a quick check of google shows Jester is very likely exactly right. Servo's go bad regularly.
(the AS3X system, or any gyro does tend to be hard on servos)
you could try plugging in a different servo into the elevator channel and see if it's the servo or the receiver...
after confirming which component is at fault I'd call horizon back and explain what's going on.

check this you tube vid (AND comments)
https://www.youtube.com/watch?v=WnHbziP5dcw