I have a 2004 avalanche, while driving my speedometer drops out or sometimes just goes O then back and forth then back to normal. can you help me solve this

You most likely have a servo motor in your instrument cluster that is going bad. The gauge needles are actually moved by small electric servo motors and if one of these starts to fail you'll end up with what you are experiencing...an erratic gauge or gauges. The fix is to remove the cluster and have it repaired. There are several companies that can do this but I take the ones we do here to our local GM dealer and have them send it out for us. I believe they replace all the servos and illumination bulbs when they fix them but doesn't hurt to ask just to be sure. I always double check the problem first using a scan too just to be sure that something else isn't wrong but it's most often the cluster that is at fault. Hope this help you out!