My G5 won't boot anymore, it halts at grey screen and if I then wait some minutes (about 2) the fans spins up and become very loud!

The same if I boot from a MorphOS 3.11 cd.

And the same with a Lubunto live dvd, I boot with in the firmware prompt with "live video=ofonly".
The last lines are "Caling quiesce...." and then "returning from prom_init", then after some minutes the fans spins up;(

I can get into the bootmenu and firmware.

What could be broken?