You probably have a poor connection to the instrument cluster, it's very common. If your scan tool will read the cluster module, it usually will show a "loss of communication" fault. When this happens, the gauges may drop, and the warning lights may come on. Everything will still be running normally with the engine when this happens. Next time it does it, smack the top of the dash and see it the light goes out.
The fix involves removing the cluster and cleaning the male pins in the back of the cluster. You will see little black spots on the pins where they are corroded. You need to scrape the black spots with a small screwdriver blade. There is a new repair cluster connector available at the dealer (assuming they are still in business!)
Hope this helps!