Strange.
Can you run a checkDB on that database? See if there's any form of corruption.
Do you know if anyone there does direct updates of the system tables? check if updates are allowed - (exec sp_configure ) and see what the value of allow_updates is.
Gail Shaw
Microsoft Certified Master: SQL Server, MVP, M.Sc (Comp Sci)
SQL In The Wild: Discussions on DB performance with occasional diversions into recoverability