Need coolant advice

[phanatic32212]

I hope no one gets mad at me since this is not a truck question, but this is beginning to worry me and I know you all have alot more knowledge than me. So, here goes:
I have a 1999 Chrysler Sebring 6cyl. The engine has been running very hot lately. If I sit at a stop light or in traffic for more than about 30 seconds, the temp guage jumps to almost overheating, BUT if I turn on the heater while all of this is happening, nothing but cold air. As soon as I start moving the heater blows air hot enough to bake bread and the temp goes back down. I don't have any rough idle. The car runs fine except for that. I don't have any leaks and there is plenty of coolant.

Thanks for any insight.
Jake