I'm pretty sure it is a Firefox issue or in relation to the scheduler. Go into your Admin-Event Log and disable some of the events (under the arrow drop down). Also try searching Google for "Firefox memory about:config" and you should see some stuff about fixing FFs' memory leak problem that comes from minimizing and maximizing the window and stored sessions. I've also experienced this problem much more on Vista, especially with FF. Hope that helps. Also, try turning off AJAX in DNN. I also had to turn it off in Catalook. It was causing too many problems with links, buttons, dropdowns and postbacks. One other thing I did, which I don't know helped was set my performance in Host to memory.