[Matt B]

Interesting comment about the vacuum, I'll definitely check that.
Are there any "usual suspects"?

Also, I already was intended to ask you gentlemen about the following - maybe it relates to each other? When I run her at part throttle, I'd say somewhere between 5 and 20% throttle, I hear a hissing noise pretty much from the center of the dash, I'd locate it below the front window defroster outlet.
It's frequency doesn't vary by engine revs neither by how much throttle I apply, amount of throttle only changes the intensity of the noise.
It's pretty loud, very easy to hear. I already checked the engine vacuum connections and fixed the dual PCV grommet. Noise is still there.

As I said, the malfunction of hot air being blown into the car with maximum fan speed (and HVAC in "OFF" mode) happened during extended WOT conditions in a mountain run.

You think these things are the same animal?