It has nothing to do with the powerjack. The only thing the PJ does is LOWER the voltage going to the receiver. It does not AMPLIFY the voltage to the servos....one of two things going on here:

1) The voltage is not set right on the regulator

and / or

2) Welcome to Hitec servos......I lost 4 of them (at different times) on my 40% AW Yak.....bye bye. Back to another trusted brand.

Chad