It could be many things you would want to do basic tests to figure out what is going on. You could have a vacuum leak, a leaking EGR valve, or a failed Idle Air Control Valve. Without carrying out basic tests any answer would only be a guess. There is a company that independent auto repair shops get their information from, this same information is available inexpensively for people that work on their own cars.. The information is year make and model specific, covering repair procedures, torque specifications, fluid capacities and specifications, service bulletins, component locations, wiring diagrams ect.... Alldata is very easy to navigate http://bit.ly/AllData_Repair_Manuals_Online
Thank you very much Mr. Patrick. I just replaced the EGR valve 2 days ago, and have replaced the Idle Air Control Valve myself about 3000 miles ago. I had to measure it precisely to get it to work properly. My Tahoe ran fine until just last week after a long haul, (1000 miles). 3000 miles ago, I replaced both cat converters, muffler, 1 of 4 O2 sensors, #5 fuel injector, cleaned the air intake flow thingy (all metal), top dollar plugs wires rotar and cap, all vaccume lines fixed and tested, many fuel filter changes, all fluids changed, tore apart rear end inspected and put back together with purple king limited slip fluid, and multiple engine diagnoistic checks by a few different mechanics. Now that I have writen all this out, I think I will check the vaccum lines again, but I have a feeling the Idle Air Control Valve needs tweeking. Is it possible that thing moved a bit? I will check the measurements again tomorrow, I think it is 1-5/16 inch, from flange to tip of cone. This truck now has 148,000 miles and runs like brand new pretty much, what a truck, I tell ya.