5D mk3-Liveview not working correctly on external monitor

[nickad]

Hello,

I use my Canon 5D MK 3 on my Octocopter for filming and taking stills. I had it set up to take pictures using the intervalometer
with ML. All was good. I had the intervalometer shoot every 10 seconds with a review time of 2 seconds which gave me a liveview
through a downlink to a ground station of about 6 seconds. I was having troubles with my HD-SD downconverter so I bought a new one. Since installation, I cannot for the life of me get the liveview to work. It will show a picture that has been taken for a second then the screen goes blank and flickers until the next shot. I have taken it off the octocopter and I am testing with an external monitor and it still does the same thing. It works fine on the cameras display, with no monitor plugged in to the mini-HDMI. I have switched cameras to an Canon 5DMK2 and the same problem.
Could it be that the mini-HDMI on both cameras are damaged? (unlikely, I suppose but not impossible) Is there some sort of menu selection that I could have made that would cause this? I have tried using the ML 'HDMI- Standard definition' setting and this does
not help.

Any help would be greatly appreciated.

Thanks