(presumably by not reaching the required lines) thus breaking the site; again, I can't confirm a direct correlation, but as soon as I removed addthis, we stopped having issues.
Anyway, the were seeing in IE 9; we were experiencing similar symptoms in Firefox and Chrome too, at least, but most often in IE. As I say, I've removed the addthis code from our live pages, but I saved the source from one and posted it as http://www.storkie.com/addthistest.html - you can visit that page to see the problem in action. Any advice or solutions you can offer will be greatly appreciated.
Webpage error details
User Agent: Mozilla/4.0 (compatible; MSIE 7.0; Windows NT 6.1; WOW64; Trident/5.0; SLCC2; .NET CLR 2.0.50727; .NET CLR 3.5.30729; .NET CLR 3.0.30729; Media Center PC 6.0; .NET4.0C; .NET4.0E; MASA; BRI/2; .NET CLR 1.1.4322)
Timestamp: Tue, 6 Mar 2012 16:47:41 UTC
Message: Object doesn't support property or method 'getElementsByClassName'