I have a couple of Apple Power Mac G5 machines that I have deployed as servers: they're running GNU+Linux, connected to the world only via an ethernet cable.

When I need to restart these machines, they will hang at some early stage; eventually the fans will switch to full-speed and the noise will draw someone's attention and they will call on me. I then connect a monitor and keyboard, restart the machine, and it boots fine. Further diagnosis shows that **the machine will not boot properly *unless* a keyboard and monitor are connected**.

This has nothing to do with MacOS, since it's not on the machine any more. It *also* has nothing to do with GNU+Linux, since it hangs very early, before the point where the OS is involved. Is it a BIOS setting? (*Update 2009-11-29*: morgant's answer suggests that it could be a failure in the boot loader; I'll investigate this.)

How can I change it to **boot correctly without monitor or keyboard**?