That sounds like a powertube oring issue. If it didn't leak before you changed that oring, then your powertube spacer is probably ok, so try a different oring there. You can confirm if it is a powertube issue or an on-off issue by holding the trigger in. If it shoots and then sputters when the trigger is held, then the problem is the on-off. If it shoots and then there is no sputtering when you hold the trigger in, then the problem is in the powertube.
Except for the Automag in front, its usually the man behind the equipment that counts.