Welcome Richy to the HDTimeline forum. While others will probably weigh in shortly, I suspect the problem is in the display terminal connections having a bit of corrosion or slight warpage due to aging of the plastic/installed PCB, which will be shall I say a rather "vigorous" amount of work accessing it without damaging things and elicit a ton of expletives.
If you get access to the LED/LCD display, check the solder connections first for bad or cold solder joints, and if display is directly soldered, probably nothing you can do. If push on connector, carefully unmate the display carefully, you do not want to distort the terminals or bend the display as they are very delicate. It may work, but the best solution is to use contact cleaner from Radio Shack, mating and unmating contacts followed by a bit of dielectric grease to prevent further corrosion
In HD' defense, the displays are about $15-20 and are generally not made to be serviced. The overall guages are not made to be disassembled and require tons of patience. You might look in the service manual, but likely does not have the detail to break down the assemby. Good luck and tell us how it comes out.:coffee