IE Favorites Will Not Open After Time

While in IE, you drop down the left pain of favorites and click on many everything is ok until about 5 minutes or so of clicking around the favorites then all of the sudden you cannot access them. Then when you close down ie the minimized selection of the last page you were on has to be closed via ctl-alt-del. If you go back in IE same thing its good for a bit then no access to favorites again. Any idea?