I'm having the same problem with the computer not shutting down. Goes black for a while and then reboots. I'm on High Sierra 10.13.5. It started today after I bought a new 4K monitor, also the stretched apple logo on boot has come back even though I have the CsmVideoDxe-64 driver loaded. In the GUI menu of clover I can only set the screen resolution to 2560 which is obviously not 4K, not sure how to fix that. More concerned about not being able to shutdown though. Any ideas?