my 98 f150 xlt 4.6 ltr i changed the plugs auto lite plat II, cleaned the K&N drop in filter-not oiled. replaced the iac. new maff. so why does it idle changes every 4-5 seconds a thousand or two in rpm difference. i notice it even when im driving down the road. really notice it at stops. does anyone know what im talking about and then on cold starts down the road at stops engine wants to die and hard to keep it going at acceleration. drives great when truck is hot. when the idle is higher at stops even makes the truck move forward. HELP