hello all, my bro built this computer for me about 3 years ago, haven't had really any problems with it until about 2 or 3 days ago when it shut down out of no where. it's an AMD Phenom 2 965 BE (deneb) w/ a HD radeon 6850 graphics card. not sure what the problem is.. it just shuts down instantly, no blue screen, no error messages, it's not over heating. i checked all the wiring and even replaced the power supply and it's still doing it. there are no shorts, everything runs perfectly normal then just shuts down instantly at random times. sometimes it'll stay on for 5-6 hours, sometimes won't stay on for longer than 10 sec... any idea??
Since you've already ruled out the PSU, I would look for something grounding out. Or perhaps a failing device such as a video card. If the system has on-board video, try running the system with just that and the discrete card removed. See if you can get the system to stay on past its normal failure point. What other devices are in your system?