Recently when on holiday in Italy my 5DII stopped powering up. Turn the switch on and - nada. Pulling the battery and popping it back in brings the camera to life, however as soon as the camera goes in to sleep mode, or I turn the power off, the problem returns. Sometimes the camera shows Error 80 at this point:
Shooting is not possible due to an error.
Turn the camera off and on again or re-install the battery.
I've seen various posts on the interwebs about pulling the clock battery and leaving the camera for a while - have tried this and also replaced the clock battery to no avail.
Has anyone else had this problem - did the body have to go back to Canon for repair or did you manage to fix it?