Could be one of many things. May be it's dirty ICV

(idle control valve) this ICV or IAC can be cleaned

out w/carburator cleaner, but I'm suspecting you may 

have vacuum leak.

You should first try this procedure.

Turn the igintion on and open the hood and listen for

a sound of air leaking( "whistle") some where around

the engine area especially valve cover of gasket area

or small hoses.

If you can find the where the air leak is than spray with

carb cleaner where the sound is comming from and listen

for any change of RPM. 

if its not the vacuum leak than could be battery itself

or O2 sensor.

Your interior light goes dimmer and gets brighter is

normal during rough idle due to low amps of battery

altenator.

Also it depends what type of vehicle it is.

excuse my english

cheers!

JC

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