things to try:
(1) Buy a shielded HDMI cable, OR buy a ferrite core that fits snugly over the cable and attach it to the existing cable
(2) Try replacing the HDMI to DVI-D adapter
(3) Try replacing the HDMI cable itself
(4) Try switching which monitor is DVI-D and which is DisplayPort, to see if it's just the one monitor on the one port, or whether the flickering follows the monitor, etc