g5 freeze after startup and blank screen

Hi, I have a Powermac G5 Dual, its a few years old and I previously had an issue with the screen (line patterns and lank screen) which was serviced and in the end they replaced my ATI Radeon graphis card which seemed to do the trick.

Since updating to Leopard OS 10.5.6 whenever I start my computer up I get the chime etc and i can log in on my password screen, but straight on the desktop the mouse keyboard everything freezes and occasionally this is followed or preceeded with a blank screen. The fans then Rev up pretty loud. Additonally i do not have any 3rd party RAM

Ive tried a lot of stuff such as

resetting PRAM
trying to boot in safe mode (gets stuck on white screen with apple logo, doesnt boot)
running fsck -fy

Can anyone help?

Are you the original owner of this G5, or did you just purchase it recently?

Just wondering if you purchased it recently...maybe the G5 came with problems.

- Nick

no im the original owner, bought it new from apple directly