did you remove the barrel ? clean her well while apart ? make double sure the barrel is seated COMPLETELY into the receiver and the magazine cap good and tight ?
remove screw in choke and clean the threads on choke and inside the barrel ? fouling can leave deposits in the threads if the choke tube had ever worked loose . it may feel tight , but in reality it is not into the barrel as far as needed .
just a few thoughts i wanted to throw at ya ,,,,,,might save you the price of a new barrel .
by the way ,,,a friend of mine has a mossberg slug gun . it started shooting erratic . installed a new barrel and it still doesn't shoot like it did when the gun was new .
hard kicking , higher pressure loads cause a hammering effect where the barrel and receiver are joined . my point is , even if you spend the money on a new barrel , it may or may not solve the problem .
hope it works out ,,,,good hunting