I had some flaky gage problems when I first got the car--even misdiagnosed by a Chevy dealer. I finally traced it to the electrical connector on the back of the gage cluster. The electrical contact just rests on bare tabs on the printed circuit on the back of the cluster, so any little crud/corrosion over the years can mess up the connection. The solution was to clean the contacts with some contact cleaner (little spray can from Radio Shack) and scotchbrite pad. For good measure I stuck a little piece of foam--stick on weatherstrip stuff--behind the gage printed circuit contacts to hold them up for batter contact with the wiring harness connector.
Hope this helps someone else--took me a while to find the real problem. I chased all over the cars wiring before I found it.