The trigger popping out of place as you're closing it up is normal with a lot of mechboxes. PITA...but normal.
If you've reassembled everything and it goes from Safe, not being able to pull the trigger because the bar is blocking it....to Full auto and it's shooting, and you can go back and foth between the two without issue, then the the issue lies with the operation of your cutoff lever (it's the bent bar/lever that sits behind your sector gear). That lever is tripped by the lobe under the sector gear with each rotation (shot).
There's little spring (outside of mechbox and attaches to the selector plate) that keeps the lever reset in the down position, ready for the next bump.
The lobe kicks it up, the nub on the inside edge of the lever bumps the notch on the switch block and flicks it out from between the electrical fins of the switch body. The spring attached to the switch body flicks it back and it stops against the little post behind it, ready to be picked up by the trigger again.
You don't have to fully reassemble the gun when you're troubleshooting. You can simply close up the mechbox, screw on the grip, do the motor. When the selector plate is fully forward you're on safe. Anywhere back from safe to where it's far enough back so that it is not touching the cutoff lever it's on semi...when the selector plate is holding the cutoff lever back (which holds it up out of position inside the mechbox) it's on full auto.
If moving the selector plate back and forth results in the proper fire selection...then the issue lies with the reassembly of the mechbox into the body. Ensure that the selector plate is in semi (i.e. not on safe and not on full...won't stay on full on it's own anyways)...and that the selector on the body is in Semi. That way the lobe that's attached on the inside of the body to the selector will be oriented up...and that it'll slip into its spot on the selector plate nicely (vs. jammed onto it or whatever).
Best of luck
Maybe it's just me, but reading your post is hard and a bit confusing. Adding in a bit of punctuation or separating some of the points helps a lot with deciphering it and providing a response. More detail is always better as well.
Last edited by m102404; November 29th, 2012 at 10:52..