I have been working on a 01 YZ426, It runs like a top for 15-20 minutes. then starts misfiring and stalling. You can restart it 4-5 times then it requires a new plug before it will start. I've heard rumours of defective carburetors, bad CDI boxes, bad coils. The plug is pretty black when I take it out. It is re-jetted for altitude-7000ft. Any help out there?