DR. DAVID GRAHAM IS LOSING WEIGHT AGAIN. His wife noticed first, then his colleagues at the Food and Drug Administration. Graham is a skinny man, and when he drops weight, his cheekbones seem to sit higher on his face. His striped cotton shirts, the frumpy uniform of a government scientist, hang more loosely on his narrow frame. But he isn't eating, and no wonder: Graham, the scientist who brought the Vioxx scandal to the nation's attention, feels like a marked man.