Hi all. I have two Mac Pros. The first one is a 2009 Mac Pro 4,1. I applied the 4,1 to 5,1 firmware patch and replaced the 2.26 GHz Quad-core Nehalem processor with a 2.66 GHz Hex-core Westmere CPU. It is running a PC 1GB ATI Radeon HD 5770 which I have flashed with the Apple EFI ROM as per the instructions on Netkas. The second is a 2010 Mac Pro 5,1. I have replaced its 2.8GHz Quad-core processor with a 2.66 GHz Hex-core. It is running the Mac version of the 1GB ATI Radeon HD 5770 (which I, obviously, did not have to flash). Both computers are running 16GB of RAM (2 X 8GB). Ever since I upgraded to OSX 10.12.1 Sierra, both of these computers exhibit the same artifacts in the EFI boot screen. When the progress bar is about halfway across the screen, a horizontal line of garbled pixels (about a quarter inch thick) appears about 3 quarters of the way up the screen. Then, the screen goes black for a fraction of a second and then the boot screen reappears sans the garbled pixels and continues along its merry way. Other than these visual artifacts, there are no other symptoms on either computer... no crashes, no slowdowns, no video artifacts, no loud fans, nothing. When they were running El Capitan, neither computer exhibited this behavior. I am thinking that this is a software problem probably having to do with the video drivers contained in Sierra. Has anyone else experienced this and, more importantly, does anyone know how to get rid of this nagging problem?