id4gun,
Clearly, your problem and solution lies in your post
No need to guess much
Its a problem with the main screw holding the metal unit in place.
I am assuming that you have properly fastened the screws in your above complaint. Get about checking that, and this time, you can use some screw-locking agent. This will fit the screw in a little more tightly, and is advised for springers.
Can you tell if
1. It is the side screw or the bottom screw that keeps coming off?
2. The threading on the screw is bad.
Two side screws and one bottom screw (the one in front of the trigger) hold the metal unit in place with the wood. If any of these screws is not fit in well, the recoil of the spring will make the metal unit go amiss.
This could be because the screw has lost its threading. In which case, you will have to replace the screws and fit the unit well.
Otherwise, if the threading on the metal unit is bad (you will know this only when you remove the stock apart from the metal unit), then it may require a bit more work. You may sometimes fit in a slightly oversized screw, or you may have to re-do the threading on the metal unit.
Hope this info helped
Is that really need enough? I can use a couple more really's there! 
