I keep getting a cylinder misfire code on cylinder 5 only on my '07 5.3. After I drive it 50 -100 miles it goes away and comes back in a week or two or any time my wife drives it. I have replaced the coil pack on that cylinder to no effect. I am about to replace the plugs and wires as it has 102K. Also, it idles at 500-600 rpm and the stabiltrac light comes on. Is this system electric or vacuum operated?. I can rev the engine too 800 - 100 rpm for about 10 seconds and the s/t light will go off. Thanks in advance for input.