I don t know how old the Barrington Stage in Pittsfield, Massachusetts (in the Berkshires) is, but it cannot possibly ever have jumped as wildly as it has been jumping, up and down, back and forth, in the current production of Ain t Misbehavin, the musical about composer Fats Waller and his Harlem Renaissance music. This is a whopping good musical, a jewel in the crown of musical theater in America, and at the Barrington Stage, in the Berkshires, where so many New Jersey residents vacation, it is just terrific.