X server fails on HDMI when TV input selector changes

I have a Mythbuntu box connected to a Toshiba Regza TV. When I boot the box I have to manually select the HDMI input in the TV or I get no display.

Under previous versions of Mythbuntu I would wait until I was at the main Myth TV screen after which I could change the TV input selector switch back to one of the other inputs (Scart 1, Scart 2, Digital TV etc.)

At a later point I could then switch back to the HDMI input and the MythTV screen would reappear.

However I recently upgraded to 14.04 and now when I switch the TV input then return to the HDMI input there is no signal from my MythTV box which means I can only use Myth by rebooting the box.

As I could switch my TV inputs without a problem using previous versions of MythTV I suspect this is a bug in the X server.

n.b. I have not performed any BIOS updates, or changed any hardware. The only change in the system was the upgrade to Mythbuntu 14.04.