Issue with @@SERVERNAME exceeding 30characters

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

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.