If this is what I think you are talking about, my '03 does it too. Really noticeable on cold starts. Here's what happens... I turn the key and wait for the engine heater light to go off. The same time the light goes off, I hear a slight click from the engine compartment. I think it is the heater going off. I start the truck and turn on the dash lights to check guage readings. After a few seconds, I hear the same click, the dash lights dim slightly, and the idle changes. A few seconds after that, click, lights come back and idle changes again. This repeats for about 3 minutes, then it kicks to high idle and runs smooth until I drive off. The change in idle really seems to be tied into the block heater cycling... at least on my truck. Hope this gives you some more ideas.