I have a relatively new truck that's idling rough. If I'm sitting still, it stays at around 500 rpms, but every few seconds the needle dips and goes back up. I've been to the dealership several times, but they keep telling me it's normal. I have another chevy truck that doesn't do this. One line they're feeding me is that the engine deliberately misfires one cylinder while it's idling to do some sort of diagnostic check every few seconds. Is that true? If not, what else can you suggest as a cuprit?