Originally Posted by cronk
Ive seen some higher mileage 5.3s develop intake gasket leaks which could cause a vacuum leak on certain cylinders intake runners, but that would probably present its self more at idle or low speed. Still something worth looking into if you dont find any valvetrain issues.
Spraying a little carb cleaner along the intake while idling and if the idle picks up speed suddenly, you found the problem.
Usually when stone cold....Once they warm up, The intake expands enough to seal up the vacuum leaks.
Unrelated, But.....Replace the bolts/grommets when doing intake gaskets on these! The grommets collapse over the years & many intakes are replaced because of it. It's not Ford 4.2L V6 level notorious, But still happens.