Lutz,
At times the ground and scenery seems very dark bacause the sun is very low in the sky. Once the sun rises it is very easy to see objects on the ground. If you fly around the same time each night try a different time on the weekend, (+ or - 6 hours), and see if the problem persists.
I have an Asus v3800 TNT2 and had similar problems to the ones you are experiencing. It's a function of the server clock affecting how fast the sun moves, (take a look at the clock on your dashboard).
It is being looked at by the crew at HTC.