Engine stalling in very hot weather

[serendipity]

Our 2008 4sp automatic 1.5l Yaris sedan stalls intermittently or loses power at low rpm in very hot weather (>35C). Car has 150k kms and has been doing it since around 80ks. Car is completely stock.

The most common set of circumstances is after car has a long period of idling or lots of stop/start driving or slowing down after a long period of highway driving. It starts again immediately with no issues. If temperature is less than 35C then doesn't seem to be an issue.

No engine codes have been recorded. Fuel filter in the tank has been replaced recently and resulted in no change. Throttle body has been cleaned. Spark plugs replaced. Car uses E10 fuel.

To my inexperienced mind it seems like it may be a vapour lock although it does not appear to be related to amount of fuel in the tank.

Another (possibly unrelated) symptom is that when driving with a very light throttle setting in a 50kph zone, the rpms seem to randomly drop for a fraction of second from 2000 to 1000 rpm and then back up to 2000rpm. This symptom happens at any outside temperature (hot and cool weather). The drop is so short that you barely notice in terms of the cars driving but you can see it on the tacho.

Any help would be much appreciated!