I'm "running" a 2008 Mac Pro (3,1) that's refusing to boot any OS or from any HDD. If I attempt to boot into a Mac OS/Recovery, it begins the start-up process (as indicated by the loading bar under Apple logo) and continues to slowly trudge along up to nearly halfway before it reboots. Sometimes it includes the obvious error screen stating 'Your computer restarted due to an error.' If I attempt to boot into Safe Mode, it freezes on the bootup screen just after removing the load bar. If I try to boot into Windows, it freezes on a black screen. I've tried removing components one by one, including: Keyboard & Mouse, Various HDDs (I have two HDDs from which I may boot + other storage), RAM (Pulling them, reordering them, etc.), Capture Card (BlackMagic Intensity Pro), and I'm working on getting access to a replacement GPU to test. I've also tried resetting NVRAM and SMC. There have been no changes. I'm at the end of my rope. Please, someone tell me it isn't what I think it is. I've been on the fence about upgrading my GPU for years due to cost. I don't want to have to buy a new motherboard as well. Please, someone save me with a cheap fix!