2003 Nissan 350Z Overheat at high revs

Vehicle does not overheat in traffic or at lights, drive above 2500 rpm for a short while and car then starts to overheat, however, if I turn the heating system on and turn the fans up the temp will return to normal no matter how high the revs are, if I turn the heater off the temp will rise again

I have changed the Thermostat and the coolant fluid, I have done a sniffer test to see if the HG is blown, all ok

Any ideas welcomed