What radio system are you using? If its a Spektrum/JR DSM sysytem and you have a JR amplified Y-harness on those servos, that is your problem. The Amplified Y harnesses with stsyem DSM on higher than 4.8V will cause that. Simple solution is just replace them with non amplified Ys.

The 8411 is fine on 6V or 6.6V. That isnt an issue at all.