I think there may be some memory leaks in DHTMLXCombo component with IE.
Everytime i refresh a page which includes a combo, memory usage in task manager increases (While leaving the page). This can also be tested in home.wanadoo.nl/jsrosman/) which showed some leaks.
Do you have any workarounds or updates?
If you are using loading from XML, there are some non-fixable behaviors in IE6 which cause memory leaking for AJAX operations.
In case of IE7 there is no persistent memory leaks.