After testing today everything is good to go and the gun locks tight with no signs of blowing open. Even tried higher velocity shells in addition to the ones I was having the problem with.
Just an update. The shotgun is working fine and not experiencing any problems. It has already bagged 4 pheasants and a squirrel.
I wouldn't give a second though about picking up another Fox or Stevens 311 if the price was right. It may not be a $1200 Browning or Ruger but the pheasants today couldn't tell any difference.
I have a feeling that I could install the original parts and this whole issue was just a matter of a top snap screw being loose. Doesn't help you need a custom offset screwdriver to tighten it.