I recently was having a similar problem on my SRT coupe. When starting out driving both sides would blow cold, as the engine temp got up to normal the drivers side only would blow hot. After another 3 to 5 miles the left side would begin to blow cooler and eventually get back to cold. I disassembled the duovalve and found it to be quite dirty. Steel wooled all the metal parts and spent a lot of time removing general crud. Seems to be working just fine since. I would like to drive some more to be sure but so far so good. The are several YouTube videos you can watch.