[wrightz28]

Shot in the dark here:

When you say the heater is on, are you using any settings that utilize the a/c (defrost, flr heater?). Either way, what it sounds like is for some reason the a/c compressor is on, the ECM dosen't see it and thus it's not adjusting the idle air solenoid or can't adjust it. The compressor shoudn't be enough to kill the engine but it would put extra load on it and drop the rpms.

Just a thought. Maybe the control panel is bad on the climate control? If you have it on heat can you hear the compressor clutch "clicking on and off" or just visually look for it, because from your narrative, your rpm s/b steady then drop the go back to steady if the ECM is not adjusting the idle air mixture.