flickering headlights,dash lights

05 gmc jimmy head and dash lights flickering, battery is about 3 years old and the altinator new this summer. the flicker is quite rapid.

I would be looking at bad battery cables, grounds, dirty, corroded, not tight, etc..

Good advice. Also check the alternator connections, including grounding through the mounting bracket.

I am going to wager that the flickering only occurs while the engine is running, and at about a 30 Hz. rate. It you're not looking closely you might not even notice the flickering. Is that correct?

I have seen this in a couple of instances, and using an analog voltmeter the voltage fluctuation can actually be observed. There is a chance that the alternator regulator is in oscillation which can be caused by poor connection of the E terminal, field terminals, or a failing rectifier in the alternator bridge. There is also a chance that the alternator brushes and slip rings are sticking or not seating well.

thanks guys, apart from the battery and alternator, where else should I look for grounding.

If you haven't found the problem yet, there is a GM TSB that addresses this issue and takes care of the problem. Very easy fix and cost is under $10. A few posts down in this link, is a copy of the TSB:
http://chevyavalanchefanclub.com/cafcna/index.php?topic=131131.0;wap2 Connecting the new voltage sense wire to the underhood fuse panel, instead of the rear of the alternator, will give far better results, and brighter headlights ;)

I would suggest a boneyard for the alternator connector, they usually only charge a couple of bucks for it.