I think IE7 is at times unstable, there are still a number of instabilities in it, but i suppose you've got to let the IE team settle back in, so i can only assume it will be optimised and improved in IE8.
I've been noticing however with many browsers in conjunction with websites which heavily leverage AJAX technologies, that at times it causes the browser to lockup or freeze, and i'm only presuming the model is the same, but i guess it's probably making way too many async requests to the backend, and then requesting the browsers main UI thread to update the DOM, now i'm only assuming the browser has one UI thread, and if that's the case that sure makes it sucky.