I have a question I should have asked a while ago... How many polys are in this scene?

Also, try deleting the history for all objects if you don't need this. This will free up a lot of memory. You can also run File>Optimize, and set things accordingly in there. It should flush out some of the extra crap hanging around.

If you want me to take a look at it, shoot me a PM, and I'll take a look.