How old is your bolt spring? Have you changed it lately? They do wear out and can cause bolt stick issues if worn and weak. If your sear is a bit worn it usually starts off with a leak right away but it is possible that movement of the oring and compression during the first few shots causes it to manifest later. If this is the case, then a shorter powertube spacer will fix your problem.
Does the gun stop leaking when you hold the trigger in? If it does, then the leak is definitely in the powertube area. If it doesn't stop leaking, then the leak is in the on-off area.
Except for the Automag in front, its usually the man behind the equipment that counts.