It does seem like its running out of air. When you fire it, does each of the fired ball have the same velocity, or does the velocity drop for each successive shot? What type of air system are you using? After the gun stops firing, is there still pressure on the trigger, or is it limp?
Sometimes, the ASA pin will depress the bottle regulator pin too far and choke off the air supply. There will be enough air in the supply lines to get a couple of shots off, but that is all. If this is the case, you can put an oring in the ASA to keep the bottle from screwing in as far.
Except for the Automag in front, its usually the man behind the equipment that counts.