I've noticed something that might help: the wierd textures and especially the lighting problems seem to happen most when I change maps with the changelevel command or by beating a level. The next level ends up all screwed up. However, if I use a clean "map e2m3" command, for example, things look fine. Perhaps that will help in locating the problem; hopefully it won't complicate it further.
There's one other thing I've noticed. It's not necessarily a bug, just something interesting. When I quit DP and then run it again later, I usually get to see the last frame the game was on when I quit, while it loads. In other words, it's like the game took a screenshot when it quit, and then it shows it to me as it loads the map. Neat. Is that an intended feature or a "Microsoft feature"?

When my computer inevitably explodes and kills me, my cat inherits everything I own. He may be the only one capable of continuing my work.