NetBoot and Intel iMacs

Hi, I'm having a lot of trouble with some new Intel based Macs we have just purchased. I have a working NetBoot 10.4.9 server that has been used many times before to image PPC Macs. I've created the image and install-set. The iMacs seem to get a DHCP address, but pause at the Apple Logo with the spinning cog. it doesn't get as far as the GUI screen. I don't know where the issue is as my PPC Macs are getting through to the GUI screen fine. Has anybody experienced this before? Any solutions or insight would be greatly appreciated.