my jeep will rev high to 2rpm when first starting it up. i turn it off and restart and it acts normal. it has been known a few times to rev high while in park. so i shut it off and back on and it does fine. also when driving the rpms will fluncutate up and down and the engine feels like its missing. i have replaced the idle sensor. it still does it. the check engine light doesnt come on. im at wits end! please help.