How much air movement do you have going over the heatsinks?
100F with a lot of air moving is going to keep it cooler then 80F and dead still air.
=Cooling=
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If you run a stick miner, use a cooling fan.
Increased cooling can reduce the chip's overall power usage.
Non cooled sticks are subject to thermal runaway where heat will trigger increased amperage draw increasing heat in a cycle until the unit fails.
So make sure you have enough air moving over it.
Also, make sure that the heatsink is tight. Heating and cooling over time could have caused the screws to back off a bit.
-Dave