Strange goings on in climate control.

Evening all, I've had my Octavia for 8 years now, an excellent car. However the climate control is behaving rather strangely .I have the temperature settings at 16 degrees both sides normally on auto, the problem is after about 10 minutes driving the air coming out of the vents suddenly gets hot. If I turn the heater settings onto the screen it stays hot.I have to turn the settings to off then on again for it to stabilize...any ideas ta.