Check to make sure you don't have some broken teeth in one of the elevator servos. Sometimes engine vibration can cause one with broken teeth to slip. Also, if it has a couple of broken teeth they may only be apparent within a specific range - so when flying, you hit that "area" on the gear train and it can't hold its position.

Sounds like both elevator halves are not in synch to me. Considering it was flying fine and didn't do this until the outside loop makes me think something happened on that specific flight. Might be something other than the teeth but... is this two servos on the elevator or just one? What kind of linkage is it if its just a single servo?