Hello, as a rule the first thing to suspect with this kind of a condition is a problem with the IAC (Idle Air Control). Sometimes they will get carbon build up on the pintle and that can keep the pintle from moving in and out as it should. The IAC is what controls the engine idles speed by allowing more or less air into the system as needed. But it can also sometimes be just a bad IAC or the IAC could be not getting the correct signal to it from the computer because of another sensor that may be defective and giving the computer false information. The only way an IAC can be reset is with the use of a class 2 scan tool. But you already have a pretty good idea of the things to check for if you have been looking for vacuum leaks. I would inspect the pintle for carbon build up and if that doesn't appear to be the case you can take a look for some of the other things that GM suggests posted below. Other than that, you will need to have the IAC tested with a scanner to see if it needs reset or is getting the wrong signal to it. Thanks
An intermittent may be caused by any of the following conditions:
Caution: Unless directed otherwise, the ignition and start switch must be in the OFF or LOCK position, and all electrical loads must be OFF before servicing any electrical component. Disconnect the negative battery cable to prevent an electrical spark should a tool or equipment come in contact with an exposed electrical terminal. Failure to follow these precautions may result in personal injury and/or damage to the vehicle or its components.
Notice: If the IAC valve has been in service: DO NOT push or pull on the IAC valve pintle. The force required to move the pintle may damage the threads on the worm drive. Also, DO NOT soak the IAC valve in any liquid cleaner or solvent, as damage may result.
Cleaning and Inspection Procedure