Another heater question.  Curiosity, I guess.

My heaters seem to work.  I have some leak in the back that I can smell
briefly whenever I turn on the rear fan.  Other than that, all is well.

But!  When I pull up to an intersection with a light or a stop sign, the
temperature of the heated air starts dropping immediately.  If I sit for
more than a minute or so, it seems to be blowing room temperature air.  Not
as cold as what's outside, but not necessarily heated, either.

Meanwhile, when driving the temperature gauge stays down near the bottom
mark.  When I stop and idle, that needle will eventually start to climb,
'til it nearly touches the warning light.

Then, when I take off again, the temperature gauge goes back down, and the
temperature of the heater air goes back up to nice and toasty.

What's up with all this temperature change?

Matt