I have no proof that 6v is the definite problem. I'm only taking a guess comparing your's and David's set-up that have had failures vs. mine that has not. May very well be an incorrect guess.

You can get 5.2v JACCIO regulators from Central Hobbies.

We originally started using 6 volts years ago prior to digital servos. With digitals I do not believe we need that much voltage, but I do like the consistent supply that the regulator provides. That consistency was more important in pattern competition then in jets, but it's still nice to have. From what I have seen, 5.2 volts is plenty for the digital servos.