Hmmm...I don't know exactly what's causing your problem, but in the meantime, I found plenty of other flaws in your level that you ought to fix.
1. Needlessly high amount of sectors. You ought to merge any sectors that have identical heights, flat textures, and effects; it would help the loading time.
2. WAAAAAY too many vertices! I can tell that you really, REALLY want the level to look like it has smooth curves. But you could probably cut out half of the vertices, and the level would still look good, not to mention load faster.
3. I saw a few Unknown Thing Types. Any particular reason?
I apologize that I can't help with your big problem, but you ought to heed what I've mentioned until we can find a solution for it. It might simply be that the level is trying to render too much at once.