Temperature dial problem on Toyota Highlander 2003

I have a recent problem with a temperature dial (knob) control in Toyota Highlander. When turning slightly from the coldest position towards the warmer one the hot air is suddenly coming even when the dial is within the blue or white region. This occurs more often when the dial is turned relatively fast. If after that I turn it back to colder very slowly the colder air is restored. The same behavior when AC is on/off. Any advice would be appreciated. thanks.