Cannot boot Power Mac G5 normally

I have a Power Mac G5 Dual 1.8GHZ.

Here's what happens :
- I press the power button
- I hear the apple boot sound
- Nothing displays on the screen... I have no idea if it does something or not afterwards.

Here's what works :
- I press the power button and hold it for a long time (It boot up, does it's long beep, does the boot sound), then the Open Firmware screen appears.
- I type "mac-boot" in the firmware and it boots normally (I can access OS X).

I would like to restore the default behaviour of it booting normally without having to boot it from the firmware every time.

I wonder if anyone could give me cues as to how to debug this problem.


Thanks !