Based upon your comments, the issue continues in different browsers (Opera and Chrome) and unless you're sharing a single plugin through out it's unlikely the fault is with the browser itself.
However, the reason why is still broad, especially as it is intermittent.
It could be your Internet Security (if using) - it could, for example, be detecting some images as banners and trying to block them which could have this layout issue.
It could be the site you're trying to view is currently updating or there is a fault.
It could be the connection between yourself and the host is not stable and during some requests, only some of the information is transmitted to the browser. You can test this by pinging the destination IP continuously for a minute and check how many packets are lost.