I am chasing something similar..In my case the passenger side gets cold with no warning after 15 minutes or so of driving.

My Heater control valve also looks new and shiny..Previous owner said he replaced it. Since it has the elec connector on the side closest to brake booster, I know it is not OEM (OEM has it on the engine side). And the OEM unit was not shiny. I haven't replaced it yet, but I hear bad things about aftermarket heater valves.