Several things going on: super-bright lights traveling on the level, combined with very sharp horizontal cutoff (usually projectors) and LOTS of foreground light. When you go down a hill, your forward vision is suddenly reduced because of the hill's blocking effect, and your pupils aren't open wide enough to see the little bit of reflected light above the cutoff. Going up a hill I don't understand, unless the lights have dynamic autolevelers that try to keep the headlight beams perpendicular to the earth's center (bubble level); in that case, with the same car, the headlight beams should RISE when going down the hill.
In any event, I don't seem to have the problem with my factory HID (reflector) DOT-compliant headlights, aimed at 8.4"/100', or the truck's reflector e-code lights that are aimed the same way. If I did, I'd think about getting a small fog light mounted (just) on the curb side, point it straight ahead, and trigger it with a mercury switch set to come on when the car is pointing more than "half a bubble" down. The idea of AFS is looking better every day.
