|Any Errors or files being referenced as it is trying to shut down? |
Does it run in Safe Mode?
Look at the motherboard, check for bad capacitors:
If you think it isn't hardware related, I'd uninstall the Catalyst software 1st to see if that helps.
If Catalyst software removal doesn't help, try unplugging all your internal & external peripheral devicess like CDs, DVDs, Floppys, secondary HDDs, external devices etc......leave the main HDD plugged in and video card plugged in. Then test to see if it shuts down.
Then add in one hardware component at a time back into the testing process, and try shut downs each time.
MOOOOooove! OH and you're welcome!