I own a 2003 Denali and the speedometer/rpms gauges are only working some of the time or they freeze half way??

Suggest you get this fixed asap before some cop stops you and you give him the lame excuse it does not work all the time as you didn't know you were doing 65 in a 35 zone.

You should take this to your local dealer. There they can check to make certain that the instrument panel cluster (IPC) has the latest software installed. If the software is up-to-date, the IPC will need replaced or at least removed and sent out for repair.