My 2012 Tahoe, when stopped at a traffic light, varies in rpm from 500 to 600 rpm. Once I accelerate, all is well. I have replaced spark plugs and wires, mass airflow sensor. Still with same problem. I have 151,000 miles on 5.3l v8 using regular gas. On earlier models, there is an idle control valve, does this model have one? Other suggestions?