I used to have this when playing on my laptop, it was a problem with the radeon 9200 series (and now i guess the 8800s too), where polys would dissapear from the sky dome leaving a black triangle.
Try DLing the shIV scene.dat mod as it fixed the problem for me (its in the forum somewhere)
here...
http://www.subsim.com/radioroom/showthread.php?t=117628