96 Probe GT: Heater problem

I"m having a strange problem. I've noticed that my heater will start blowing cooler and cooler air whenever I"m at a traffic light or idling for any length of time. This even seems to happen when I"m slowing down at freeway speeds or exiting the freeway. ANy clues?

I've checked antifreeze level and its OK (although I have a slight leak, can smell antifreeze odor when parking in my garage at night ...have not been able to 'see' any obvious leaks or puddles).

replace your coolant thermostat.... its stuck open