I have a 1999 5 speed 3.8 V6 Camaro that throws heat while driving but when at an idle it blows cool air.

try flushing the heater core both directions...probably a restriction...dexcool plays havoc in the heater cores....
kevin

I'd suspect a problem with a diverter door or vacuum control in your heater box. Almost sounds like when vacuum is high (at idle) a diverter door is operating and cutting off your heat.

If the heat comes on slowly when driving then it may be something plugging up the heater core.

Try this. Shut off the car while leaving the heater control in the high-hot-heat setting. Wait about 20 or 30 seconds and then flip the controls to off-low-cold. Do you hear a little squeek as the vacuum bleeds off? If not you may have a vacuum server control problem - leaking hose, leaking control valve, or ruptured diverter servo diaphram.

had the exact same issue on my 99 durango. warm air at driving, cold air at idle. I unhooked the heater core hoses and stuck a water hose in one end and flushed out all the gunk, and it was nasty gunk coming out. My heater is back to 100% after that.

Bad thermostat.

Mark