Artie, I use the same exact gun, and the same Nitro 4x5x7.5 shot its #301a. My shells are red, but I bought them when Nitro was still in N.Y. So maybe they've changed something since. But I've never had one of their shells jam up in my Mossberg. I'd definitly give them a call. Because obviously your gun is patterning better with the Nitro.
I'd also like to add gun wise, have you tried pump racking the gun a little harder? When my Mossberg Deer Gun was new I'd get jam ups if I didn't rack it hard enough until it broke in a little. I also placed a few drops of gun oil like Break Free in the slide rails both by the pump, and inside the chamber. So try that first.