I'm animating a simple fly-through of a tunnel. I've got the tunnel modelled, and I've got a motion path set through the tunnel that the camera flies along. Everything works great, until I try to render it. When I render, sometimes some of the frames of the animation (like the attached frame) have large black areas in them. Most of the frames do not, however. Why is this? Could it be that the camera is sticking partially out of the tunnel? When I watch the camera travel along the path in the assemble room, it appears to be totally within the tunnel no matter what angle I look at it from.

A note on the render: I haven't even begun doing lighting yet, so I just put a criss-crossing glow texture on the tunnel to make it easier to see what the tunnel looks like and where the frame didn't render.
Thanks for your help!