I was told (always a bad way to start a sentence) that the NG wing was redesigned to allow for a higher cruise mach number - while the NG is obviously cruising somewhat faster than the classics supposedly Boeing wanted to raise MMO as well over its current 0.82. The problem this posed was that this would have required a complete airframe recertification... If this is true this would explain the relative speed instability of the NG in cruise - it definitely wants to fly at lower AOA!