You might try changing things in the options menu -- these suggestions come from the GB Glitches forum:
There are several options available under the in-game settings - lower AA, Texture Filtering, Mip Mapping, Shader Quality and Vertical Sync to their lowest settings. Also ensure Dynamic Shadows and Reflections are disabled.
The only other thing I could think of would be to make sure you've done everything described in the walkthrough up to this point, just in case there's a small hidden trigger somewhere -- did you read all the emails on the computer, look at all the notices on the bulletin board, that sort of thing.