circular hallway question

I'm creating a circular room with a circular hallway surrounding it, seperated by hallways to and fro. Unfotunately the circular hallway freaks out and does not show the floor in places, in other places you can see the floor, but fall through. And in other places the geometry is off.
The circular hallway has flat areas, areas with gentle slopes, and areas with straight drops. I've tried a straight tube with added geometry, a single clean subtracted cylinder (vertexes manipulated for height).

Any ideas? What am I doing that is making it freak?

Thanks.

Thanks. I got the hallway to work. It seemed that the vertical drops were the problem. What I did was to raise one section of the floor up and pull the next section along the hallway underneath it for the vertical drops I neded. I pulled the bottom pairs out a little bit and it seems to work now.