I think it's the aux fan temp switch.
I believe your temp switch has 2 temp ratings.At lower temp, the fan kicks in low speed, and at higher temps the fan kicks in high speed.
I think it is controlled by a resistor in the switch.
If this is so, your switch is not kicking in the aux fan at high speed at high temps.
I may be wrong, but I have an Euro model car and it has only 1 temp switch rating at which the fan kicks in - HIGH SPEED.
I have seen on Fastlane that the temp switches in the US have 2 temp ratings - 1 for low speed and 1 for high speed.
Wonder if anyone else can help confirm this?
My 2 cents.