I've got a 4,1 that is seriously driving me insane. Some background; cMP, Early 2009 (4,1), Quad core 2.66, 32GB RAM (4X 8GB sticks) I was using it a few months back when it suddenly died. Middle of work and it shut down. Couldn't get it to power back up. Checked out the forums, did a little research and figured it was a wonky PSU unit. Couldn't spare the time to get it fixed at that time, so I left it and just worked on my MacBook Pro instead. A couple of months later, figured it's time to get it back up and running. Plugged it in and it worked! For a while before it shuts down again. Did some trouble shooting, opened it up and checked out the internals. It would boot up but then shut down after a few minutes. Noticed that the fan in front of the PSU unit was not spinning when powered up. Made sense, the cMP was going into thermal shut down I thought. Sourced high and low for a replacement fan which I finally found and replaced it. No dice. The cMP would boot, run for awhile before shutting off again. Its the PSU I thought. Ok, so be it, lets just get it replaced. Purchased a new PSU and gleefully I replaced it thinking my issues are over. BUT NO!!! Still having the same issue!!!! It'll boot up, run for awhile before shutting down again! Could be the new PSU I got had the same issue (it is suppose to be brand new thou). I can reach out to the seller, but I'd like to eliminate all other possibilities first. I've tried the usual troubleshooting steps, reset everything that could be reset (NVRAM, SMC, RTC) removed all peripherals (USB except for keyboard and mouse), no PCI-E cards other then the stock video card (NVIDIA GeForce GT 120), removed RAM, cleaned out every last dust bunny and its mother. I even followed the steps in the 'Apple Technician Guide' and brought it down to a minimal testing configuration. I even took off the heatsink, cleaned out the old thermal paste and reapplied new paste. Even did that for the north bridge chip heat sink.... Even tried to restore the firmware by creating a 'Firmware restore CD' as per apple instructions but am unable to do it as the cMP does not stay on long enough for me to effect that restore. I'm seriously out of ideas. I'm sure its something stupid and simple I'm missing. I'd really like to get to the bottom of this. Any and all inputs appreciated. Thanks guys!!