As far as I know, it's related to video drivers and graphics card settings. Black lines can be removed for Nvidia cards by enabling something called "clampmap" (or similar -- google is your friend). ATI cards don't have a fix handy (I've got one, and have the lines, too). It's been a pain in the arse for the Q3W crowd for ages.

You can also load up IoQuake (alternative Quake build) and see if the lines are present there -- should be fixed there. Not much help for running Generations, but it's nice to be able to take screenshots without the lines.