Chevrolet Repair: Speedometer/Millage error, digital speedometer, caprice classic

Question
Why would the mileage read error-flashes on and off "error" mileage ect., loses speed reading per digital speedometer?
This is a '90's model Caprice Classic.
Was told some time ago the light switch shorts out and causes problems with the dash. This may be the case as when we turn the lights on the speed goes away until such time as the lights are again turned off.
What do you think?

Answer
HI JOHN, DO ALL THE OTHER LIGHTS AND GAUGES WORK OK WHEN THIS HAPPENS AND IT IS JUST THE SPEEDO ITSELF ??? IF IT IS JUST THE SPEEDO ITSELF THIS SOUNDS LIKE A PRINTED CIRCUIT FAILURE ON THE DASH FACE OR THE NXT POSSIBLE CAUSE COULD BE THE SPEEDO PICK UP ON THE TRANSMISSION ITSELF. THE FACT THE THE ERROR MESSAGE IS ON,, THE PCM KNOWS THERE IS A PROBLEM,SO THE PICK UP ON THE TRANS WOULD BE MY GUESS OR WIRING PROBLEM TO THE PICK UP..IF NO ERROR MESSAGE CAME UP THE THE PCM WOULD HAVE A SIGNAL FROM THE OUTPUT SPEED SENSOR ON THE TRANS THINKING ALL WAS OK...LET ME KNOW IF YOU NEED MORE HELP,IF SO PLEASE GET BACK TO ME. IF YOU ARE FINISHED WITH ME PLEASE RATE MY ANSWER.   THANK, YOU TOM..