The problem i'm having is when its hot, I find that the kill switch doesnt always work
This is called "dieseling" (also known as "detonation" and/or "pre-ignition"), and caused by excessive heat. See
here. To say the least, its not good. I've experienced it with my 35cc saw after small rip cuts. Essentially, the engine gets so hot that it ignites the gas even with the electric cut off to the spark plug. It means you are running the saw much too hot, and can cause long term damage.
To avoid this, first, stop working it so hard since you seem to be pushing it to its limit. Next, let the saw rest during a long cut, as in, just let it idle mid-cut with the internal fan allowing it to cool itself. Maybe a minute and a half or so of rest for every minute of hard milling (give or take).
Then,
do not try to shut it down immediately after finishing the cut. Again, just let it cool off at idle for a couple minutes. Don't just turn it off thinking it'll cool down, because it'll actually get hotter if switched off immediately after and/or during a long cut. And for that matter, don't allow yourself to run out of gas during a cut either.
Also, be sure to use gasoline with a decent octane (above 88 or 89) as octane is what helps prevent this (contrary to popular belief, octane does not give more power, it just burns at a hotter temp).
Further, make sure you have your oiler turned up well enough for the chain. A lack of chain oil causes friction and heat. Also, in some saws, the oil tank is up against the engine, acting as a coolant, so don't run it low on oil either.
And be sure to run the saw rich in the carb. A lean running saw is not what you want for milling or hot temps.
Good luck!