Try here for your manual:
And you are sure the Air Chamber oring has not dislodged itself or is missing?
This is the most likely senario for what is happening.
If so check the inside of the body of the gun for anything the bolt could be hanging on. Look for unusually heavy wear inside the gun body and barrel breach.
If the gun is firing as soon as the mag is inserted firmly i would look to make sure the trigger assembly is screwed in right/rigidly attached and that the sear is not overly worn or loose.
The fact that the sear IS engaging and the gun is not "burping" or going full auto, leads me away from that as a possibility but check it out anyway if no other option fixes it.
PS. Also what have you lubed it with? Using something too thick could be causing the bolt to stick forward.