Joe, GM had lots of gauge problems in the trucks of that era. The gauges are all electronic, each individual gauge has a little stepper motor on the circuit board, and those were prone to failure. There was even a recall back around 2007 or thereabouts to repair gauge problems based on year & miles. Google 1996 GM Gauge Problems, or 1996 GM Speedometer Repair to find a bunch of links to discuss the problems. Hope this helps you find your gremlin.