GMC Sierra rough idle

I have a 2002 gmc sierra 1500 5.3L with approx 70,000 miles. When I crank the vehicle in the mornings and drive for a couple of miles, I notice that when I stop, the idle pulsates. This tends to happen on mornings where the temperature is below 50 degrees. I had some check engine codes come up. P0171, and P0174. These are both bank 1 and bank 2 too lean. I took it to a mechanic and they said it was a loose snorkel tube. Thought everything was fine. Pulsating has returned, but no check engine lights. The truck feels like a muscle car with an oversized cam. It lurches against the brakes when I am at a stoplight. I have heard that it could be an intake manifold gasket. $$$ I am hoping someone can give me some other options.