I have an 89 Sierra, 5.7. auto,When I first got it it had very hi idle. I replaced base plate gasket nad idle air control valve. It ran fine for about a week. Now it idles rough , especially in drive with brake on. I have replace plugs, wires, cap and rotor, Fuel filter . No change..any advice appreciated.No service engine light and no vacume leaks that i can find..