I trust you've shut off the cold air intake. I would also suggest looking at the recirc damper. (The earlier versions have a lever that breaks. The later ones have a lever that works loose.) See the Brickboard archives. I think you can get to it through the glovebox.

I also had this problem and replaced the thermostat although my temp gauge read low. Now the cabin warms up quickly, but after a while things grow tepid. I suspect the cabin temp sensor. If so, this will be sensor #4 (FOUR!).

-Punxsutawney Phil