Upload & Sell: Off
I can't figure out what the heck is causing this. Well, back to Canon it goes! I'll let you all know what happens.
I should update my story: indeed, Canon took a look at the camera an replaced the main PCB. The fault only appeared *after* Canon serviced the camera the first time, so clearly I would say it was their responsibility to fix the camera.
Fix it they did, however as is often the case with Canon's repairs, they do right but often get something else wrong in a rather ham-fisted way. Case in point: the above lamp issue. This time, upon replacing the board they considered it an "out-of-warranty" goodwill repair as they declined to warranty the work.
This is incorrect on their part as they broke the lamp in the first place, during the in-warranty servicing.
But since the camera seems to work fine now for the 300 shots I've taken with it, I don't feel like spending the time arguing with them about it, given the value of the camera. If it were a 5DIII, I would take it up with them again.
Throughout the whole thing, I only paid to ship it to Canon once, which is good.