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.