I have a 1989 GT 175 J175STLCEB that starts and runs great right up until top speed. At just over 5000 rpms it will begin to bump and hesitate. Carbs have been rebuilt and fuel pump changed. Plugs look good when I get them back to the house but I haven't had time to try a high speed shutdown to see how they look then. Stator, timer base, and pack are all relatively new. My best guess is a failing ignition coil or a spark jumping to ground through deteriorating wire insulation. Any suggestions on how to begin diagnosing this issue are appreciated.