I agree with Clark on the deep clean of the barrel. It will really help.
I shoot a 20" Mossberg 835, and I shoot the 3.5-2-6 Winchester XRHD's, with a .675 SumToy choke, and I got a best of 156 in the 10" circle. I know the numbers are not huge, but the pattern is very even.
I think you should give William Lambert at SumToy Chokes and see what he can do for you. He is a great guy, who makes a superb American Made product!