Short version:
When I drive for more than a short trip (10-15+ minutes), I find my car spitting coolant out if I stop and shut the car off immeadetly. If I put it in park, and idle for 30-45 seconds it doesn't spit any coolant out, mabye a drip or two.

Also, when I'm driving and stop moving for 4-5 minutes I lose heat in the car. For example last night I went to a drive thru, and there were a couple care in front of me. As I was waiting, the heat from my heater slowly cooled off and eventually there was no heat coming from the vents. As soon as I pulled away from the drive thru heat returned.
Any thoughts?

Long version:
I have a 1999 V6 Pontiac Grand AM, and a couple of days ago I got in my car and started it. The warning for Low Coolant came on, so I turned it off and checked. It was low, so I added some, and then thought nothing of it. This did seem odd, but I couldn't remember last time I checked it, and I do live in Arizona, so Summers are hard on cars.

When I talk about the car spitting out coolant below, it's coming out of a hose that comes off of the overflow tank. The hose is about 6 inches long and dumps water onto the frame which runs off below to the ground.

I drove to work (roughly 30-35 minutes, about 1/3 is freeway), I believe the next day, and I was a little late, so I specificly remember pulling into a parking spot and hopping out without pausing at all. I came out later and there was a large puddle of coolant under my car. I also turned the car on and got the Low Coolant light again. So I put water in it, and then drove it home. I got home, stopped the car (leaving the car on), popped the hood, and then turned the car off. The car did not spit any coolant out.

So today I experimented and drove to work again, this time when I got to work, I popped the hood, waited 20-30 seconds and then turn the car off. I did this again when I got home, neither time was it spitting water. But two other times when I pull to a stop and turn the car off without waiting or spending any time in park, the car spits coolant out.

Also, as I mentioned above in the short version, I lose heat (referring to the heater) when I sit at a low RPM for more than a couple of minutes. This happened at a drive thru, but as soon as I left the drive through heat returned.

Anyone have any suggestions?