RE: Interesting problem ...input invited
Since this has happened with 2 different receivers on 2 different bands, I would rule out the receiver. You checked the battery but not under engine vibration conditions. I would just replace it and the SWITCH. Not worth chancing it.
Now , I have had 2 aileron servos do what you are describing. Randomly go full deflection and return. I found the problem to be the servo connector on my extension. I replaced it and my problem went away. Both times were in the same plane and the same connector. So, in my case probably an installation issue some where. And , visually, the connectors looked OK.
sp