<p>Something somewhat similar to this happened with one of my 5Ds last year. When Mode Dial was on Manual exposure, shifting from <OFF> to </>, the camera would not remain on. LCD Panel blanked out, came back on, blanked out again.<br>
When Power Switch was set to <ON>, the camera remained on.<br>
Problem was inconsistent. It usually worked fine with I powered on, then at a most inconvenient time, it would no longer work. I figured out a workaround to make it through the trip. Basically, I set the aperture when i first powered on, then I switch to <ON> position.<br>
I set it to Canon twice. First time, they couldn't replicate the problem. (There were other service items that they did address.) Second time, they replaced what seems to be the entire back with LCD and dial. It's been working fine since then.</p>