Need Help

[Dogtank2000]

So I have a 2000 GT all stock except for the exhaust just replaced the IAC in it because the idle jumped up and down and a couple days later I had the same problem but its when I turn the ac on and off if its off it will idle about 500 to 600 rpms and when i turn the ac on it goes to a 1000 rpms but running the ac for a little bit and i turn it off the car idles down to 300 rpms and almost and has shut off when in park. Now when I turn it on the rpms go crazy to 1500 down to 500 and just jumps all about there and makes a screaking noise and i have no clue what it would be if any one has an idea or has happened to them please let me know.
Thank you! Car also runs on premium but not tuned for it that's what the owner before put in it now so do I.

 

[TIGGER]

On 2 of my 96GT’s I had the low pressure sensor go out that is on the accumulator. When the heater was on off or on vent the cars would idle normal. On anything else they would surge. May or may not be your problem. My cars both idled normal when the heater was off. Yours seems to idle low.