It sounds like you have an A/C and idle related problem. when the engine gets really slow, the A/C pump will not create enough flow to cool the car.
So,you need to find out why the idle is getting so low, try turning OFF the A/C and then driving the car, if the car drives and idles really smooth, then there is is an issue with the A/C. If the car still idles rough, then it is an engine control issue. So, here are 2 articles that I wrote about how to inspect either one of those issues.
For a 2003 ION, you will have a data streaming computer, so I would do this inspection, whether you have a Check Engine Light on or not.
http://repairpal.com/check-engine-light-diagnosis a good tech should be able to find the problem with out a code, most likely in the Idle control system.
A/C inspection article
let me know if you need more help