speedometer works at radom

This is very common on the 03-06 Silverados. The cause is the motors behind the gauges are failing, they all go out eventually. You can get it repaired here - www.drspeedometer.com and there's a DIY guide here - www.howtorepairmyinstrumentcluster.com

Same thing happened to me. I ordered the motors and fixed it myself. Took longer to get the cluster out the dash. Now I am fixing speedos on the side. If you haven't gotten fixed yet you can send it to me and I will have fixed and sent back to you the same day.