shared folder icon disappear

This is a bizzare problem. I have a Win2003 server Entx64 machine as file server with a few shared folders. continually three days arround same time, the shared folders have “unshared” themselves. Even not allow me re-share the folders(right click, no option for sharing),I have to reboot, and then show up,
I have all service packs up to date and anti-virus scaning, no any virus is detected,

help would be apprecieated