Sorry you lost your model.
The problem was not your 14MZ as the model flicked out of your first loop after you slowed the model down with an upward roll and you got away with it.
I think you have too much elevator movement. If that is coupled to a CG on the rearward side and remember the Viper has a very short rear moment arm (more pitch sensitive)the model is trying to flick roll on full elevator and slow speed (top of loop)
I have had this problem on my F1 scale pylon racers in the past (high speed flick roll at no 1 pylon)and the cure is to move the CG forward and reduce elevator movement/use plenty of expo.
When you roll inverted how much down elevator do you need to maintain level flight.If you require none or very little down then your CG is at its rearward limit and could come forward a touch to be safe.
I have not flown a Viper Jet but looking at the short coupled design I thinkthe CG and elevator movementcould be critical.

Phil.