A few precautions as well:
1) Remove any non-raven pk3 files from your base folder.
2) Delete the jaconfig.cfg and jampconfig.cfg files from your base folder (All settings to normal)
3) Also remove any mods in the Gamedata folder, such as the japlus folder.
4) Get the latest patch (v1.1) if you havent already.
5) Clean the disc, or try playing with a virtual CD image, see if that has any change.
6) Close any extra programs you're running whilst playing (Including Jams thingy) You can put them back if this isn't the problem

7) If you're using Windows XP Service Pack2, make sure the game is on your allow list in the security section.
8) If you've changed any GFX settings in your bios, such as Fastwrites, AGP speed, or RAM clock speeds, use the restore all function to restore to defaults. And dont overclock your vid card or anything if you are doing.

9) Clean out your registry of JKA stuff, complete reinstall, delete all folders e.t.c. (Get more info on this as it can damage ure PC if you delete wrong stuff.)
10) As Raiyaka said, get the latest drivers, and don't use any except the oens directly from the video card manufacturer. Or if you have them, try using your old ones (My UT2004 screwed up when i got the latest drivers) Also default all settings on your gfx control panel, turn of anti-alisaing and things like that if you use them.