What does your Mechanic stay about it? (So we are not repeating ourselves). And I hate to be Master-of-the-Bleeding-Obvious but have you checked the oil?

As well, you may wish to post in the regular Motorcycle tribe if you haven't already. Wealth of information in there too.

EASY AS PIE!!!
THis is not anything that a mechanic can fix.
It is called Vapor Lock.
When you are low on gas, the heat will evaporate your gas in your tank, thus putting less actual liquid fuel through your lines It will be mostly fumes and wont' start, or will die.
I have seen this many many times, and yes, even been a victim myself.
The only thing that you can really do about it is to keep your tank at least half full.
Sorry I couldn't be more help with a quicker or better fix. But that is what it sounds like to me.
Good luck.

Blocked fuel tank breather, not allowing air into the tank to replace fuel volume as it is used,...........although this would still cause problems at night, and in cold weather.

If it's a single cylinder bike, maybe the Cgnition Coil is breaking down when hot. Unlikely to be more than one coil faulty at a time, so not as applicable to twins, triples or fours.

Again, if a single, how many miles has the spark plug done? i never let these run for more than the recomended amount of miles. They will still run, but can cause all sorts of flukey problems.

If it's got more than one cylinder, you need to be looking at what affects all the cylinders. Water in the fuel. Very unlikely, due to it being temeprature affected, which is why i am exploring the electrical "sparks" side of things.

I've looked at some of the answers that you have been given, and the oil level check one made me laugh. the only way that would affect it, is if it was soooo low that the bike was 2nipping up" as in partially seizing, in which case it would very soon be a wreck!!! :o)

I'm not familiar with USA emissions crap that some over there sound as if they are fitted with, and that seems well possible.

I know this is a female only site, and confess to being a British male, riding for 35 years, so hope that's ok. Why was I looking in the first place? Hell, I'm a man aren't I??? :o)

K.x