A lot of people are experiencing map problems, you might try reinstalling the demo, that fixed it for me.
This happens to old Intel graphics chipsets only, as far as I'm aware. Try updating your video driver from the Intel site, and edit settings.txt and turn off pow2_textures.
Men fear thought as they fear nothing else on earth -- more than ruin -- more even than death .... Thought is subversive and revolutionary, destructive and terrible, thought is merciless to privilege, established institutions, and comfortable habit. Thought looks into the pit of hell and is not afraid. Thought is great and swift and free, the light of the world, and the chief glory of man. Bertrand Russell