SQL Server 2005 SP2 has an issue with modifying Agent Job's through Management Studio if the @@servername exceeds 30characters
For more info on the issue refer http://support.microsoft.com/kb/940269
If you do not wish to apply a hotfix or upgrade to later service packs. You can also decrease the size of the server name if standalone
and use SP_DROPServer to remove the old server reference and later SP_ADDSERVER to add the server name
Or you can also change the Virtual Name of the server in Cluster Administrator for Cluster Installations. Change the name in SQL Network Name resource.