2002 Sunfire Idles Hard and Overheats when AC is O
 

My sunfire and I are both in Chicago, hot summers/cold winters. It has 137k miles on it. Late last summer it suddenly began idling hard with the AC on, and it overheats pretty rapidly. There's a kind of sucking/hissing sound with the AC engaged too, like the car is gasping for air. No problems with the AC off.

Does anyone know what the most likely cause is? Any suggestions for how to determine a likely cause before taking it to a mechanic? I like this little car but of course, any substantial repairs will easily outstrip its total value at this point...

Thanks in advance!