Odd problem - The control module in the dash for my 99 Grand Prix that controls the air flow, AC, Heater, temp etc intermittently works. The AC portion works OK. It will blow cold on the passenger side when Dual mode is selected. However the driver side vents blow mildly cool air at best. The heater will blow cool air then blow hot then randomly go back to cool. It is as if the control module is faulty and is sending the wrong signals to the control valves for the hot and cold.

Any ideas what I could check or test?

FYI - I installed a new heater core, thermostat and flushed the entire system. No coolant leaks anywhere.

Thanks.