How to resolve an uneven idle?

[synapseuc]

Hello everyone I have been visiting this site for various reasons for about a year now and finally decided to sign up. I have a 2004 4.4 X5 that I am completely in love with. One thing that has me slightly concerned is that it idles unevenly, I would not say a rough idle but you can just see the tachometer vary slightly only while running at rest (and of course you can hear it). Also at night the interior lights oscilate slightly in intensity while idling. As soon as you give it a little bit a gas it purrs perfectly. I have done a little research and think it may be either the Idle Control Valve or the O2 sensor. Any help would be appreciated because (like all of us) I hate replacing a $150 part only to have solved nothing. Also I am new so if this is posted in the wrong section just let me know! Thanks again