SetSmooth doing weird things

I’m experimenting with the spin function, and I’ve made a 180 degree pipe bend by spinning a tube. But Setsmooth doesn’t like it, and it has strange dark areas on it, that are lined up. It’s kinda hard to explain, so here’s a picture…
http://img453.imageshack.us/img453/1280/line0hi.jpg

Any ideas on what causes this and how to fixit?

try ctrl + N to recalculate the normals.