20 bucks says it's a loose throttle cable. For all you drivers out there that like to punch it....you're gonna experience this sooner that anyone else. I wouldn't be a bit surprised if you traced the cable from the throttle body and found it to be sagging. Of course it could be a number of things, but if you have decent miles on the engine, then chances are it IS your throttle cable sagging like grandma's sour milk sacks... h: