i got new a 625mg for a b4, it jitters under light load
its not a linkage or servo-saver problem

hold controller centered, and turn the tires lightly the servo goes crazy trying to straighten them
or turn the tires, hold it, and let the controller go to center, it will try to straighten the wheels, then give up (can release the wheels and it will stay)
bump them and it will center again

have a few 645mg for scx10/electrix ruckus that dont jitter

verified its the servo by using a futaba 3003 with no problems, other than being slow and weak

hitec says it has problems with the 79xx servos with 7.2v
http://hitecrcd.com/support/service-bulletins
but the reciever outputs 5v

thoughts?
could it be a bad pot or pcb?

Not exactly sure what is causing it, but that servo can't not be fixed by an average person. Aside from replacing the gears, diagnosing and fixing a servo reminds of the sticker they used put on old radios and other electronic equipment... 'no user serviceable parts inside'. Send it in for repair.