Screen freezes after login, stuck on stripey grey screen?

Hi there,

My Mac Pro is about 2,5 years old, I'm almost quite sure my graphic card is giving up but I'd be very thankful to have some insight from you guys.

I'll also describe my experience here so it might help someone having the same issue.

1. I was watching a movie on itunes this morning, suddenly scren goes white and computer freezes up.

2. Try to force restart it but after login it freezes up on a dark grey screen. Every time.

3. Computer runs fine when restarted on Safe Mode (holding "Shift" while rebooting). It didn't freeze at all while on Safe Mode.

4. Did all the tests and repairs with Disk Utility and no problem whatsoever.

5. Every now and then still tried to restart on normal mode but same freezing happened in 80% of the time. Strangely enough in the other 20%, computer managed to go past the login screen but would freeze again shortly afterwards. And staright away when I tried to load my display settings. Always on this dark grey screen.

6. Decided to do a fresh start -> format -> reinstall OS. Was running OS 10.8 when problem started.

7. Formatted hard drive and downgraded to 10.6 again. Installation was smooth. Computer restarted and frozen again as soon as the "Welcome" video intro started. Restarted again and it worked fine but started to freeze shortly later again, it's just taking a bit longer to happen.

8. I've just run Apple Hardware Test from the installation DVD and no problem whatsoever has been found.

Which makes me think it can only be my graphic card (?) what do you guys think?

OS 10.8.4 and/or 10.6.4

3.33GHZ Processor

ATI Radeon HD 5870

32GB Memory

2TB HD

Thanks very much guys, let me know if I can provide any more detail about this.