Has anyone experienced this happen to them. When stepping on the brake (like at a stop light) with the truck in gear the RPM drops to about 500 to 600. Normal right! Every so often the RPM's jump up to about 800. Is this normal? And when I say every so often I mean every 20 seconds or so. Now I have been monitoring this for a couple of weeks and it does it when the truck is cold, warm or hot. Any thoughts? Trip to the dealership? I tried to search for a problem like this but came up empty.