We also having same kind of issue. But its not all users simultaneously, its some users randomly. When we dig we found that this happens when the VD resources highly utilized and also IE 8 is running. We tried to regenerate this with loading CPU using load generator. Even when CPU is 100% there was no issue, but when we run IE while cpu is high VD lost session. In all related logs we didnt find any unusual error.
Hope you can also see whether this happens when any particular app is running.
rgs