Going to my ex-AA days, we had circa 20-30% more breakdowns in excessive heat i.e. at the beach side in summer.
The was mostly due to two areas; general poor maintenance during service, hose inspection, seeping radiator matrix and my favourite 'I thought something was wrong because I have to top up the water every week'. Secondly the antifreeze mixture was totally wrong, especially if they have been topping up with water. A lot of people think antifreeze is to stop the coolant from freezing but it has cleaning chemicals and more importantly raises the boiling point.
Buy a coollant tester and check your mixture. Most is us would be surprised.