Heater control

[apossati]

I have a strange problem with the heater control knob on my MY2000 Griffith 500 (A/C equipped): when it's turned fully counter-clockwise (usually COLD) the air is HOT; when turned fully clockwise the air is even HOTTER; it's only when I turn it around 45° from full counter-clockwise that air becomes cold. The problem persists even with A/C engaged.
It looks like an out-of-zero problem. Does anybody have an idea about this problem?

Thank you

[apossati]

Thank you very much for your fast response.
I'll let you know ASAP.