Hey, this is exactly what mine is doing. vroooom......vroooom......vroooom......vroooom But only when the brakes are on. Other than that the idle is just too high...
you should test the tps. it's position has everything to do with how it operates. It's circuits include the closed throttle fuel cut. also read this
http://board.marlincrawler.com/index.php?topic=35469.0
This solved the problem with my truck. when I pressed the brake, the idle cut would engage and the rpms would drop. once it got to about 1000 rpm it would get power again and speed back up. sounded kinda like someone impatient at a light trying to let someone know he's getting.... 
vroooom......vroooom......vroooom......vroooom.
p.s. also, this never stored any codes on mine.
