Hi, i have the same problem with my Mac Pro 5.1. I used a PC vodeo card flashed R9 280X for few months and everything worked fine. Few days ago a tryed to install Mojave and for thi reason i had to update my EFI mac pro firmware (from 1.39 to 1.44). After this update, my mac didn’t boot anymore (chime loop). Fortunately i had the old video card (ATI HD 6870) and the mac boot with no problem… Obviously a cannot upgrade the sytem to Mojave with this card… I read that the same video card (R9 280X) flashed with the EFI rom for HD 7970 for mac works fine. I’ll try. I hope that my experience could be hellpful for resolve your issue…