I used to have vsync turned on (official U and test U's), lfs resolution was at 1280x960@100Hz while desktop was 1600x1200@85Hz. Was not possible to reproduce intentionally but happened occasionally, when least expected. I should also mention that I always use shift+f4 and shift+f9-12 for switching window and fullscreen.
After talking to Vic I figured I'd try without vsync for a while, but also changed desktop res to 1280x960@100Hz and I haven't had one single black screen since then, and it's been over two weeks now. Maybe I've just been lucky or it's vsync/res&Hz related.
Wxp sp 2
3800+ x2
Nvidia 7800GT 91.28