@samsbike - if you were actually wearing sunglasses when you experienced the problem... I've seen discussions about polarising filters on GPS screens and sunglasses. Sometimes the orientation of one clashes with the other. If a GPS seemed too dark through sunglasses, try rotating the glasses and see if the screen becomes easier to read. If so, then you'd have to try different glasses, peer over the top of them, or cycle along with your head tilted to some crazy angle!