Hello, I wanted to get some different opinions on an issue I've been having for about a month.

I have a '99 v6 xlt. The main symptom of my trouble is that my heater will only blow lukewarm air and that's after about 10 minutes worth of drive time. It seems to be directly linked to how hard I'm working the engine (60 mph gets lukewarm air, sitting still I get cold outside air). Also, my water temp and oil temp gauges stick an jump for about the same amount of time. Occasionally the heat when it's on full will feel like it should but those instances are happening less and less. Everything I've read so far points to a blending door or a vacuum tube problem but would that explain my irratic temp gauges which is why I'm posting. What's my real problem?

Many thanks in advance for your thoughts.