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If the temp changes back and forth on its own with no air flow change, then the issue may be your thermostat on the engine and not the switch in the car. If the thermostat is opening and closing, it will have the temp changing.
Now if the air is changing in the car, see if the temp changes which each rev of the engine versus idle of the engine. If so, there is a vacuum leak between the control switch and the vent control aka actuator at the air box. That vacuum leak will need to be found by removing the dash and chasing taht whole line looking for cracks and feeling for suction along that line. It could be in the actuator itself where the small diaphragm is bad and letting the vacuum leak by.
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