Engine dies like key is shut off when engine is at operating temp, seems to be worse wnen outside temp is 80 or above. does not trigger check engine lite.

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This is normal, there is a temperature sensor. If they do not shut off after a few minutes then replace the sensor and or relay.

You said it is worse when the air is on? It is probably the ad compressor trying to lock down. The drive bearings on the ac compressor are probably almost shot causing a heavy drag on the engine

Dealer check codes

It seems strange that you have this starting/no power issues when it over 70 degrees outside but not when it is cold. When it is cold the battery and starter are normally under more pressure to start the engine and it is also harder to burn and atomize fuel when it is cold.
The reason I suggest removing the trim panel and checking voltage at the ignition switch is that is the best place to start. The ignition witch has to have power present at the switch and must distribute that power as the key is turned to operate accessories and the starter motor when turned to the start position.
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sounds like a fuel problem , water in gas , ......

they are designed not to fluctuate with engine temp and are kept stable once operating temp is reached. only if the temp exceeds a set mark will the needle rise to indicate overheating,

heat build up causing spark system to break down

tell tale sign is highway drive is fine (plenty of cool air under hood)

you may have spark module prelim failure
it'll only get worse eventually it won't run but a few minutes

is this a 4.3l

it could also be telling you that your freon is low. have it checked if its good then you have a computer control problem seperate from the engine its a control just for heat and ac.