RMD

I have an F Pace S and am consistently having the engine temp high warning light come on sporadically when driving at a consistent 70-80mph especially if going uphill on motorway. Slowing down or cresting hill and the temp comes down to normal in seconds. Anyone have similar experience and solved problem? There are no fault codes logged when this happen so dealer saying they can't diagnose the problem and therefore warranty no good!!!! Wanted to charge me £300 for the failed diagnosis. Shameful!