Hi there, I'm hoping for some help since this is driving me nuts. For a few weeks my PC has randomly been shutting down and rebooting. I know this can be caused by a lot of things but there are a few things I think I can rule out. I don't think it is a software issue, as there is no error message or blue screen, no new software has been installed recently, and this PC has never been used on the internet so there should be no virus or malware. I also tried a system restore to no effect. The problem is random and intermittent, sometimes the system will stay on happily for an hour, other times I can't even get it to boot because it just restarts over and over. However, most frequently it automatically restarts after about fifteen minutes of use. The same thing has happened before about a year ago and the problem was a faulty PSU. Installing a new PSU solved the problem so, since the behaviour was identical, I assumed it was the same problem and purchased and installed a new PSU. At first I thought it had worked since the PC stayed on for ages, but the next time I came to use it the problem was back. It may be worth noting that the startup behaviour has changed slightly since I installed the new PSU - it keeps bringing up a prompt saying "f1 for startup, f2 for bios" and waiting for a response before it goes ahead and boots which it has never done before, and also brings up a motherboard loading message I haven't seen before. This reaches the end of my PC knowhow and I'm not sure what to try next. I don't mind if I have to replace some part of my system but I don't want to replace the whole thing and I'm not sure how to find out where the problem originates. Any ideas? System specs: Windows 7 64-bit Processor: Intel Core 2 Quad Q9550 @ 2.83 GHz 8GB DDR2 RAM 500GB HD Thanks!