I tried posting this issue on the general help board, but no replies so far:
I've managed to get 10.12.6 working (sound, graphics, Ethernet) but the system consistently crashes, not on start-up but anywhere from 2 - 20 minutes later the whole system freezes. The behavior is unpredictable and doesn't appear to be related to any specific application being open. I had exactly the same problem when I installed 10.12.5 a few weeks ago, so it's not specific to 10.12.6.
I have used memtest and the RAM (2*4GB DDR4) seems to be OK.
I'm using a 4K screen over DP, but the problem remains even when I use a lower resolution.
I have spent ages reading threads trying to troubleshoot the problem, trying different SMBIOS values (iMac 14,2 17,1 18,3) and different settings (e.g. changing the IntelGFX, IG-platform-ID, inject/not values), but always end up with the same problem. I also tried copying the lilu.kext and intelgraphicsfixup.kext to the 'other kexts' folder, but that leads to a crash on boot. I am at a loss as what to try next and would be very grateful for any suggestions. Can anyone replicate the the problem using the attached EFI? Apologies if I have missed something simple - it seems that others are running Sierra on the Deskmini without this issue and I can't see what I am missing. Thanks!
diagnostic files attached.