I have a Mossberg 500 myself. I have the exact same problem. I've been shooting pumps all my life and this is the first time I have had that happen, so I know its the gun. I believe that is simply result of a fairly cheaply made gun and wear. My Wingmaster has fired just as many shells and has no trouble at all. In the gun world you get what you pay for.
I still use the gun for turkey since I only need one shot, but between that and the famous mossberg fore arm rattle I just will not buy one again.