I've got an iMac 27 2017 with an i7, the 580 graphics card and an SSD of 1 TB. I've extended the RAM to 40 GB. Everything worked fine, but the last couple of days the screen stays black on startup every once in a while. The fan is running and the screen get's black-grey (as it is 'on'), but than nothing happens. After putting the iMac off and on again everything works fine. I've already did reset NVRAM and SMC. Starting the machine while pushing D results in no errors. What can this be?