A mate of mine has a late 2008 Unibody 15” MBP. He installed the 10.7.5 update via Software Update, after the mandatory restart it kept hard rebooting at the grey Apple screen. He figured it was a corrupt install so re-installed Snow Lep from DVD, then bought Mountain Lion via the App Store. Same problem returned after the installation completed. It now refuses to boot the Snow Leopard installation DVD (have tried more than one). The MacBook passes the AHT tests. I’ve swapped the RAM with known good from my 2009 Mac Mini with no change. The Mountain Lion install on there boots fine via Target Disk mode on my MacBook Pro, and the HDD checks out fine in Disk Utility. The local Apple store are blaming the HDD. I’m not that convinced, my hunch is it’s something on the logic board, perhaps the graphics. Any suggestions?