Engine over heating

The over heating only seems to occur when the engine has been used at peak revs. Eg track use It isn't losing water prior to the over heating
The thermostat has been changed, the pump has been checked, all pipes have been checked for leeks, the system has been flushed and back flushed. It doesn't over heat if the heater is on but the heater only starts to work when the engine has been used at high revs. And will keep on working. If the heater is not on initially that is when the over heating occurs, and the heater when turned on will not get hot. The bottom radiator hose is luke warm and the radiator is cold.