I had a similar problem with a newer 870 Express, some years ago. I polished the breech, which was very rough and seemed to be "grabbing" the shells when they expanded (even a little bit) after firing. If I remember correctly, all I used was a brass brush mounted to a power drill, and a lot of Hoppes or some other solvent. It was messy, but it changed the dull finish of the breech to relatively shiny, and the problem went away. Try that first and see if it helps.
I've been told that plastic has "salts" that can cause corrosion in the breech, even though your barrel may have been regularly swabbed. But if I remember correctly, the 870 had a very dull looking breech from the get-go.