My 1999.5 2.8 A6Q has developed an intermittent heat problem. Sometimes, the heat comes on fine. Other times, when the engine is definitely warmed up, cold air pours out regardless of temperature setting, recirc/not, or whatever. The car had the full coolant flush and fill about 10K miles ago, so I don't believe that there should be any crud in there. I replaced the temperature sender since the gauge sometimes read dead cold when I knew the engine was warm, and now the gauge reading is "normal." But my heat still has the problem: it is intermittent. This morning, while in for something unrelated, I had it checked for codes, and there were none (heating system or other). Any ideas what else I can check?