Failures to extract a spent hull can cause all sorts of issues with feeding the next. Were I you, I'd try a different brand of quality ammo (STS, AA) before doing anything else.
I took the 870 to a gunsmith Monday, and he seemed somewhat puzzled about the cause. He did depress a couple of different springs in the action, and stated he thought one of them was a little weak.
No offense intended, but that doesn't sound, um, like a highly qualified diagnosis.