I've always used Chrome. I'll have a go on IE and see if I get the same problem. Seems to mainly be when I have the message window on the bar, then have a look at a thread in the forums. When the new thread has finished loading, it won't let me open the chat window.
Edit: Damn! its Chrome based... IE works fine. I'll have a look at all the plug-ins for Chrome and see if I can get it sorted. I hate using IE its so slow... lol