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.