Intermittent boot to black screen on R4

I have an R4 with Windows 10 Pro Bios A 11, stable with this configuration many  months. I did upgrade the video card from GTX 580 which came with the computer to an EVGA 1070. Starting shortly after that and whether it is related to that or not I do not know, I have the situation where I turn the PC on, the Alienware splash screen shows along with boot progress. The monitor shows HDMI signal.  As usual it goes to another screen that offers to setup RAID on my drives. I do nothing, and then it returns to the Alienware Splash screen to finish booting up. Normally it then says it is going into sleep mode, and screen goes black and in an instant I get a Windows chime sound and the Win 10 welcome screen appears and all is well.

Lately about one in ten boots, after the monitor shows it is going to sleep, after offering to configure RAID,  instead of the Windows 10 welcome screen, I get just a black screen, and monitor says "There is no input, please move mouse or activate keyboard to wake up computer" and then a black screen remains indefinitely no matter what keypresses etc I make. Waiting a long time does not solve situation. If I  reboot invariably with the second boot all proceeds normally. Fans areOrunning etc computer appears on. I have all sleep modes for the PC disabled and always have. of course without ability to visualize anything on the monitor I have no idea what if anything is going on under the hood when this happens.

To troubleshoot I have switched monitors, tried Display Port and HDMI inputs to monitors and it still happens occasionally. I have run all the extensive Dell System Diagnostics and all show "Pass". In particular there is no identifiable issue with memory or video card.

I can identify no system problem that could trigger this intermittent boot to black screen. Any ideas would be appreciated. Otherwise all the software, operating system etc runs fine.

Greg