Did you by any chance swap out the Chu wood stock for a Fiberglass?
The guide rod retaining pin is liable to work it's way sideways. The wood stock holds it in place. The fiberglass stock won't unless you fill in the area. Since that area needs to be filled in to work properly, most guys also fill in the selector cut out.
If this is happening with your origional wood stock, the cause is still probably the same. You will have to figure out why the recoil guide retaining pin is able to wiggle out enough to release the guide rod.