I am afraid IE11 and the new versions of Chrome brought a great deal of changes (including breaking ones) and our controls need to be updated in order to support them. When Q3 2012 (that you are using) came out these browsers were not created yet, so we could not have tested and prepared for them.
You can try adding an x-ua compatible meta tag to put IE11 in IE9 or IE10 mode, but I cannot guarantee this will work. As for Chrome - I am not aware of a way to revert it to an older version, and even if that is achievable on your own machine you cannot for the end users to do this.
Thus, I strongly advise upgrading your RadControls version to the latest (Q3 2012 SP1 with SP2 just around the corner).
If that is not possible you can try removing pieces of the page until you can pinpoint the problematic content/control/logic and try to workaround it for IE11. If you can send us a sample that shows such a trimmed down version perhaps we could help, but I cannot guarantee this.
If you want to get updates on new releases, tips and tricks and sneak peeks at our product labs directly from the developers working on the RadControls for ASP.NET AJAX, subscribe to the blog feed