Vent heater temperature problem

I have a C240 2005 and the problem is that since i have owned the vehicle, on occassion, if i turn down the temperature on the heater,
it continues to blow very hot air. So if i start out at 85 degrees, and turn to even 65 degrees, hot air still comes out. I then have two settings, hot and cold. If i fiddle with the knob it eventually works. This happens 10% of the time and I have taken the car in. They have witnessed the problem once but cannot find any code error or specific reason. The people that witnessed it were the intact supervisor and not the head technicians. So i am wondering if anyone has had a similar issue.
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