Unable to properly boot Power Mac G5 running Debian without a screen connected

I’ve got Power Mac G5 running Debian 6 in headless mode as a media/home server. When booting it without an attached screen, the fans ramp up from the idle speed to maximum over a couple of minutes. The red interior warning light is *not* lit. The server is ping-able the entire time, but startup utilities such as SSH and other are never started; or at least they aren’t reachable. If a screen is connected until the login-prompt appears, it works just fine. Connecting a screen *after* booting does nothing; it must be connected the entire time. I do not know, nor am I able to test if this is a problem when running Mac OS X. Has anyone experienced this before? And if so, is there a solution to this?