You should really avoid using ntext, text and image datatypes and use nvarchar(max), varchar(max), and varbinary(max) instead http://msdn.microsoft.com/en-us/library/ms187993.aspx. I guess you have old database migrated from previous version of SQL Server.

For mow you can convert ntext to nvarchar(max) before using string functions. However, I would create and use CLR function which would give way better performance for what you're trying to do.