I've got the DP cable in now and all seems good. I had tried two different HDMI cables - one really caused a lot of blanking, the other less. Suppose I could buy a reputedly hi quality HDMI to prove things out but the DP cable seems fine. Problem solved.  
I just got a new XPS 8940. I have an Acer monitor that's a year or so old. I have never had any problem with the monitor and have tested against another desktop and a laptop. No problems. With the new XPS 8940 the monitor will randomly go black during web browsing, program execution etc. Seems always to come back during boot screen presentation and then will again randomly go black once into the OS (Windows 10). I have tried both the "auto" and "Intel graphics" settings in the bios with no change in behavior. Have tried 60Hz and 75Hz refresh rates with no change in behavior (monitor properly detects refresh rate change). Any suggestions on other things to try?
XPS 8940, Acer monitor goes black using HDMI