i am having a problem with my idle. When i first start up the car it some times miss fires, but then is fine as far as running is concerned. but the idle is off some of the time, for instance, i think the idle should be at about 750 - 800 rpms, it idles about there some of the time, but some of the time it idles as low as 550 rpms, but averages about 650ish rpms. i notice that it dives down when i step on the brake, or anything that uses power, so i think the alternator might be going bad. but when i turn the a/c on the idle falls to 550 then shoots up to 900ish then falls again, it does that for a couple of seconds the catches itself. any ideas whats causing this?