Due to the Ethanol content of modern fuels, be it at a ratio of 5% or 10% or more is heat causing evaporation in hot engine bays.
When the fuel enters the engine bay it enters a very hot environment that then influences the fuel which predominantly creates partial evaporation of the fuel within fuel lines and float chambers that can then cause erratic idle and erratic throttle.
This is attributed to the Ethanol ratio in the fuel evaporating due to the heat this is due to the Ethanol having a lower evaporation point 78.37°C (173.07°F) than the petroleum which has an evaporation point of 200°C (392°F) so therefore reducing the Octane rating of the fuel and therefore its ability to combust correctly.
So, it can be an advantage to make sure that all fuel lines are in good condition and routed away where possible from high heat sources and making sure that all heatshields are in place.
There is the possibility to use heat wrap tubing around fuel lines to help reduce the effect of heat on the fuel and even use a wrap around exhaust headers to reduce under bonnet heat.
Excess heat can also be caused by a poor performing cooling system that may include a partially blocked radiator core or even the incorrect specification coolant/antifreeze being used with an incorrect coolant to water ratio.
Excess heat can also be caused by a poorly performing ignition system and/or ignition timing.
Correct engine oil grade is being used.