This took me 3 days to work this out, so be appreciative people.
Monitor would turn on and off every 5 seconds or so and switch back on with the Oculus in the Main HDMI port and Monitor in the Display port.
If the Oculus was in the the Display Port and the Monitor was in the HDMI port, the monitor would be ok on start up until you used your Oculus and then stopped using your Oculus. In which case the symptom would be flashing display inside your Oculus Headset and this would continue.
AND.. I re-installed windows too!