SID’s showing under security

We are running in mixed mode. Windows NT domain controller w/SP4. Windows 2000 servers w/SP2. We migrated an NT server to Windows 2000. We built a new server used robocopy to copy data and security from the old server to the new server. Got ridof the old server. Problem, when we go to a folder and look at the security all we see are the SID’s. This started happening about a couple weeks after the new server was put into place. If we reboot the server it seems to work again for a coupleweeks.

Any ideas?