I had the same problem when I installed that 960 Grid System extension from DMX Zone. They clearly didn't do much research on it and kind of released way to early without testing. Since I had been a fan of the 960 Grid previously, when I saw the free extension, I figured I would give it a go. Then I quickly found out that it kept causing similar problem. I think they have released a few updates since, but I just completely deleted the extension off and that solved my problem.