I have a 2009 Mac Pro flashed to 5,1. I upgraded my single W3250 with a X5690 off of ebay. The issue that came up is this: on powering on it chimes, shows the apple logo loading screen, goes up about halfway the progress bar and then restarts. I already reseated CPU and RAM (tried both a set of 24 GB @1333 and the original 6 GB 1066 ecc), reset the NVRAM (multiple times), tried with both the original Radeon HD4870 and my Radeon rx560, tried recovery mode and booting from a Catalina installer, then downgraded to Mojave and tried with that installer; i also tried removing the hdd, ssd, and expansion cards, leaving in only the absolute necessary fot it to boot at least in recovery mode. It shouldn't be a compatibility problem, right? The machine worked perfectly before. It could really be a faulty processor at this point, but it seems weird to me judging by the symptoms.

Hi, I’m trying to do the same thing but after I put everything back up and turn it on, the mac chimes, then shows the apple logo and the loading bar, which only goes up halfway before restarting, in a loop. Did you have by chance similar symptoms?