Idle Problem on 99 GMC 4.8L V8

Hello,
I am seeking some additional advice/assistance to this odd problem that I have been having recently.
My '99 GMC 4.8L V8 has been idling very poorly lately, the engine almost never dies but goes down to maybe 200 rpms catches itself and then goes up to about 800 and this continues to fluctuate till the engine is completely warmed to normal temp. The engine does this only when the outside temp is below about 40 degrees and the colder it gets the worse the idling. Also in the cold mornings is like the engine needs to be choked. I have already changed fuel filter, plugs and wires and still this continues. I taken it to a few shops and they don’t seem to have any answers. The S.E.S. (Service Engine Soon) light has recently come on and took it to a shop and they replaced the Idle Air Control Valve, this still has not fixed the problem. One shop thought that it might be a temperature sensor for the computer or something, also a possible vacuum leak. I guess I’m leaning towards cleaning the fuel injectors as a next step.

Any help would be much Appreciated.

If you plan on fixing this yourself (or at least trying to) I suggest buying a scan tool. They can be purchased at most autoparts stores. This will tell you what code has set the SES light to on, and give you a starting point. It's better than guessing and replacing a part you dont need to... Good luck.

It may be an O2 sensor, some of them have a heater element for pre-heating, and yours may not be working, but finding out the SES code is the only way of really knowing for sure, take it to a NAPA store or whatever is in your area, and they will pull the codes free for you.