I have a 2008 Mac Pro 3,1 that was bought new and has given me absolutely no issues until recently. I flashed a 6870 and everything was going great for several days. Then we had a power failure. The system was on a Tripp-Lite UPS (I had no idea about the sine wave power supply). I wasn't home, so the power supply eventually gave out and the system lost power. Now, the UPS itself is dead. It won't power on, it just lights up for 1 second and shuts off. The Mac will run fine for anywhere from 30 minutes to 24 hours before powering itself off and rebooting. Sometimes it gets stuck in a loop of reboots and sometimes it comes right back up. The issue never happened with high loads on the GPU such as Steam games on max settings. It usually happened with youtube videos in Chrome, oddly enough. I've swapped out the 6870 and put the old 2600 back in and now have no issues. So my question for those in the know is, did I kill the power supply or could it be something else? The 6870 uses two power cables and the 2600 just uses the bus.