My MacPro 3,1 (2008) has been working fine for a while, I left it on overnight (which i frequently do) but it had shut down this morning. Now when I try to boot it up, I see the front power light and fans start to spin for about a second or two then everything goes dead. When I check the diagnostic LEDs, the two CPU OT LED's flash once about 5 seconds after the machine dies. Have researched the official service guide and web sourced with limited success. Any ideas are most welcome!