I have a 2004 Sierra with the 5.3l V8 in it, and it's started not idling. It doesn't idle when I start it up, and then it will idle occasionally. It idles at about 400 RPM, but usually I have to hold my foot on the gas a little to keep it running. Besides this, the truck runs fine and runs strong. I was thinking it was a throttle positioning sensor? I read on some other forums with people having similar problems they checked the mass airflow sensor. I wanna be certain before I replace a part, I don't wanna just throw money at it. Anybody have some advice?