My guess is light bulbs that aren't making good contact, or simply on their last legs. You can pull the gauge and measure voltage on the sockets to see if it's steady or if it's as spotty as the lights are acting. I'd replace the lights regardless, considering they're 15 years old. The groovy news is that we have a guide on LED conversions, so you can switch colors
and not have to worry about replacing them again:
topic563777.html
Let us know what you find!