User Mgr for Domains doesn’t display

Windows NT 4.0 – User Mgr for Domains will quit displaying after an amount of time. After a reboot, it works ok for a day or so. If left alone for some time, can dbl clk on icon of User Mgr for Domains and it won’t appear but, Task Mgr shows it as running. What can I do to get it to work all the time without rebooting the server.