It really sounds like you have a problem, but are wanting to short cut it. Only after you have done a complete diagnostic, checking things like IAC steps, cts and tps would consider making any changes. You may be able to hack up the settings to where the idle adjustment is broken enough to give a fixed speed under all conditions. There are a lot of things that can cause the problem you are describing. Its common to have a worn IAC loose steps meaning it's not where the computer thinks it is. Also the wiring on the manifold CTS is so faulty most of the times the sensor comes with a new pigtail. Intermittently altering the cts temp voltage can affect the idle. To fix it right, you are going to have to do some digging. Someone may have a copy of the idle diagnostic steps in the factory gm manual, but purchasing my own manual used off ebay is one of the first things I do for my GM cars, if I can find them reasonable. pay attention to the details, I can't tell you how many thousands of times a customer would insist they had checked everything, and thrown on multiple new parts and really it was fixed within and hour or two with careful step by step diagnostics. The best tip I can give you is pay attention to the small stuff when checking for problems, so many times people get in a hurry and miss the problem when it's right in front of them. I have done it myself.