Ever since I got my truck my idle usually sits around 750 rpm and I know what that rpm sounds like. Here reciently the tach occasionally has a mind of its own and wants to drop to 500 rpm and then jump back up to 750 even though the sound of the engine doesn't change. I haven't had time to pull the dash apart to see if any plugs are loose and my waranty expired about 2k ago. (figures )

Any clues? Anyone else seen this?

 

Nobody have any input?