If you have a vacuum gauge, hook it up to the line going to the EGR valve. It should read 0 vacuum at idle. If it doesn't, you need a new EGR vacuum regulator (EVR) solenoid (It's mounted on the back of the passenger side strut tower), if it does read 0, you probably need a new EGR valve.
As for the A/C being on, that was just a question to consider if the EGR valve test yielded nothing. It definitely sounds like the EGR test yielded something, so you should fix that before moving on to something else.
Let us know what you find out.
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