If you select the option for Diskless NetBoot on a Mac OS X 10.5 image hosted on a Mac OS X Server version 10.5 NetBoot server, the NetBoot client may stop responding soon after it starts up.
Download and install the Mac OS X Server 10.5.1 Update or later.
As a workaround (until you are able to update to 10.5.1 or later), you can disable Diskless NetBoot.