Temp guage in the red, I'm stumped

Replaced thermostat and fan switch. Took car out on hot day and after 15 minutes at 100kmph gauge went into red. Turned car hearter back on and gauge went back to normal. This leads me to believe after all this trial and error that there is a wiring flaw somewhere. Spoke to a friend on the week-end and he has had the same problem and the only way to re-set temp gauge is turn on the interior heater and blast hot air for 10 minutes. Oh well all that for nothing. I bet Honda know about this.