Most likely, you should either clean the duovalve or replace it.
Otherwise, the issue is in the climate control module in the console.

One way to find out, is, when it acts up again, measure the voltage at the outer two pins of the duovalve. With both temp wheels at full cold, you should see close to zero volts on the outer two pins and battery voltage on the middle pin.
If you do, the valve is stuck on that side. The center pin is battery hot, the outer pins are the left and right coils. "COLD" is a fully grounded coil - less than .5 volts should be on the outer pins with the wheels at full cold.