Nitrogen filling is a great way of limiting fluctuations in pressure due to temperature changes but first you still have to get your running pressures correct to optimise contact patch and maintain as steady temperature range once up to racing loads as much as possible.
If your running temperature is to High the tyres will go off and the coefficient of friction between tyre and track will diminish. High temperatures can be caused by a number of factors such as , High track temperature, incorrect compound for track or configuration of vehicle , incorrect tyre section/profile for configuration of car etc etc. The most common cause for historic cars though is to low a pressure which results in the tyre working too much. Imagine trying to snap a plastic ruler, you keep on working the rule in one spot until the polymer at the point of stress reaches a temperature where it becomes soft and the fails, its the same for the polymers in tyres.
If your tyres are maintaining a temperature of around 85 degrees C this is roughly the right running temperature around 110 and above and the tyre is over heating and you are loosing grip.
I hope this is of help
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