Whenever my car is going under 20km/h and i am not applying any gas the idle becomes very unstable and starts jumping from 200rpm to 1000rpm violently and quickly. It gets even worse when i press the brake. I took it to a mechanic who replaced the IAC valve, which helped, but did not fix the problem.(Yes i will be talking to him tomorrow )

(Side story: about 3 weeks after buying the car (About 10 months ago) my car had this problem, but REALLY bad, I would have to ride the gas slightly anytime i was going slow or was stopped. I replaced the IAC valve myself, it seemed to help for about a week, but now has the same problem again (Almost seems like the IAC is getting worn out or something REALLY fast))

I have gotten advice and ideas from MANY people. Nothing seems to be helping, no mechanics around my area have any idea of how to fix it. sadly...
I myself work at a Autobody shop and am skilled with bodywork, refinishing, ect. but when it comes to the motor I am not the person to be there.

I just want to be able to drive my car without having to eye the RPM gauge every time I'm going slow. This is my daily driver which i need to be reliable.

 

Do the idle learn procedure and see if that does the trick. Can find it by using my name as the author and idle + learn in the keywords box.

RBob.

 

Does it behave like this in Park/Neutral during idle as well, or only when its' in gear...?

 

Well when I had this problem before I did the idle relearn which somewhat helped but did not fix the problem. Also the closer my car is to operating temperature, the worse it gets.
While in park the car seems to be stable. Idle gets a little rough while at operating temp

 

Have your mechanic test your CTS, MAP and O2 sensors for accurate readings...

 

No check engine light????

Do this again step by step just to be sure(IIRC this is from Robs old post):

Disconnect battery, wait one minute, and reconnect.

Turn off A/C.
Turn off Backlight Heater (rear window defroster).

Do Not Touch The Throttle at Any Point.
Do Not Touch The Steering Wheel at Any Point.

Set parking brake and chuck tires.

Start engine, let settle a bit and put trans in drive.

Let engine run until fan cycles on then off.

Turn off engine.

Wait five minutes.

Start engine and place in gear.

Let idle for five minutes.

Shut off engine.

End of procedure.

 

Idle Relearn seemed to help this time, idles better. thanks