Networks

I have a server 2008 domain controller and have redirected domain windows 7 workstations desktops and my documents to the server which works fine. However randomly the users will lose their desktops and need a logoff/logon to restore them. Have checked all the usual suspects.. network computer issues etc but can not come up with a practical answer. I have many other windows XP machines in the same environment for a long time and have never had any issues this is only occuring on the new windows 7 computers?