My truck idles at about 600 rpm sitting still, but once I start driving and take my foot off the accelerator (approaching a stop sign or just slowing down for a speed limit change) the rpm drops to only 1300 so the truck coasts at about 35 mph until I apply the brakes to reduce the speed then it drops to 600 rpm. Also, in the above scenario where I am coasting at 1300 rpm and 35 mph if I shift into neutral the rpm quickly drops to 600 rpm. Any idea what is wrong?