Display problem, stripey shading

Can anybody help me understand why shading is displaying like this? It’s happened before, but this time restarting rhino and/or the computer isn’t helping. Shows this kind of striping in any display mode that uses “ambient occlusion” shading…

Just an update. Of course, as soon as I restart after posting this, everything is fine. But if anybody knows what causes this, I’d still love to know…

Wow, nice woodgrain effect - but no, no idea what that might be… I’m glad it’s gone anyway. If you can reproduce it, shout, though.

-Pascal