Is this a new problem on a new site or it just started happening on an existing site?
Have you installed all the .NET service paks and windows updates?
Are there any javascript errors happening on the page? use the browser dev tools to look for script errors in the javascript console
There is no known bug in mojoPortal but there are some known issues with IE 10 and asp.net that are best solved by using .NET 4.5 hosting. I know that MS has fixed the known issues in .NET 4.5 but I'm not sure about .NET 3.5.
It is time to move away from .NET 3.5, we may stop supporting it soon and I've been recommending for a long time for people to move to .NET 4.5 hosting.