I also am thinking that it has something to do with JavaScript. In particular, I think something in
switchcontent.js is probably causing the error, from what I've read on other sites (specifically
here and
here). The thing that's causing IE to throw an error seems to be that you're putting this line
<script type="text/javascript" src="/scripts/switchcontent.js"></script>
before whatever code the script is used in has a chance to load. Firefox won't complain but IE 6/7 does. It has "window.addEventListener" (I saved the Previous Updates page to disk then did a text search on the .js files) which seems similar to the stuff on the page in that first link. No other javascript you use has that.
From what I can gather, IE complains because you shouldn't be able to modify something
before it's loaded. Which is also why the error is so random - it depends on your connection speed. If the page tends to load too slow, it will probably throw the error. But if it's faster, it might not.
I'm not completely sure if that's the problem, but I'm posting it anyway just in case.