vapour lock
The problem has all the symptoms of vapour lock. Check that the line from the tank to the carb runs everywhere well clear of hot parts. If you have an inline filter installed it should be somewhere where it gets free cold air flow, so not behind the cilinder or something.
If the bike runs great in the mornings and evenings there'll be nothing wrong with jets or settings - the higher temperatures in the middle of the day would maybe make it run a little rough at idle, but would certainly not make it cut out at speed as if running out of gas.
Did you install an aftermarket tank? They tend to reduce 'ventilation' around the cil head / carb area.
You could consider to wrap fuel lines and filter, if installed, in aluminium foil to keep the heat out.
Luck!
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