Everyonce and a while my car will start to idle hard (rpms fluctuating anywhere between 400-1200rpm) and the lights on the dash and my headlights will dim as the rpms get low. Could this have something to do with the batter or alternator, or is it something else? thx-Eric

 

To check if your alternator is ok you need to check the battery for 12v with the car off then 12v with the ingition on, then with the car running you need to check for a climb from 12v to about 13.5 to 14v when almost instantly, after that you need to turn lots of lights, sterio, aircon and anything else electrical on then check to see if you still around 13.5 to 14v, there will be a small drop but not to much.
A loose belt will also look like a failing alternator.
Anyone plese feel free to correct me if im wrong.

 

If your idle is dropping to 400 rpm the alternator will not put out enough voltage to charge the battery, the headlights will be enough of a load that you'll notice the drop at night when it falls below 750 rpm, if you have eliminated the airpump and are running the under drive pullies then you will notice the drop at a higher rpm than 750 rpm. Around town driving will wear on the battery as typically doing allot of restarts and little driving will not replinish the battery and over time it will weaken.