I doubt it's a shell stop problem, usually they feed two, not one, and tie up the action.
First thing I suggest would be to take it apart and clean/light lube everything. While doing this, see if either action bar shows a bend,excessive wear marks, or even peening at the tip.Naturally, the inside of the mag tube gets cleaned, and maybe even the steel wool/dowel/electric drill treatment. If no damage is evident, put it together and see if the problem persists.
Also, Greg, how old and how much used is that 870?