EDIT: I just tried the exact same resolution in zdoom 98, and everything else turns black except for the intermission screen... here are the two pics for comparison
(sorry for the crappy pictures, but photobucket sucks, and resized only one picture)
ZDoom 98: [spoiler]

GZDoom 0.9.20: [spoiler]

BTW just incase you need em.
Celeron 3.06GHz
1GB DDR 400
nVidia 6200 256MB AGP