A worn bolt spring causes bolt stick by not pushing the bolt back far enough, but usually not full auto. If the bolt does not go back far enough, it doesn't allow the sear to rotate forward to open the on-off pin, so the front chamber won't recharge with air for the next shot.
Full auto is generally caused by a worn bolt or sear, or an on-off top oring that is leaking. I would replace all the orings in the valve as a first step with any used ebay or interent purchased valve unless it is coming from a known and trusted source. That way you know you are starting with good fresh orings. That reduces the chance of an oring related malfunction. Once that is done, check for proper operation.
Except for the Automag in front, its usually the man behind the equipment that counts.