Did you check to see if it has spark and fuel when this happens? They make a small tool you can rest on the spark plug wire and it will show you when it sparks. If its not sparking consistently when you run into this condition at all check the ignition coil and wire resistance/ohms first. The heat build up in high gear could be causing resistance to skyrocket, and eventually cause a no spark condition. If the spark is there but random and erratic it will be either the stator or CDI box. I chased a random spark condition for days on mine, which made me think CDI box, because my stator ohm'ed out well within spec. A brand new CDI came with no fix, so I gave up the $500 for a new OEM Yamaha stator. Bam, now it starts perfect with a sniff of the button, and runs w/no problem. Start with the cheap fix first (spark plug, wire, cap, and coil resistance) than dig deeper, I have also had to fix many wire breaks in the harness that were the result of prior hack jobs, or years of work hardening. Often times a corroded connector or ground is also culprit.
