Surely its the same problem as Darren Dev Id'ed - ie COM5 Channels like Film4+1 & QUEST+1 ....
"I've spoken to a specialist today and we're currently investigating an issue only affecting DTT channels on COM 5 - skipping over ad breaks with adverts that are broadcast in mono can sometimes cause the box to crash and reboot. Does that match your experience? i.e. were you watching any of those channels?"
This is also happening to me on two boxes, the older recently died with a hard disk failure, but was suffering from total crashes when fast forwarding through a recording. Powering up via the the remote shows BT logo and you-view screens, same as a mains power on. I've only noticed this happening within the last month or so.
BT sent me a replacement T4000, which I then manged to cause the same crash shortly after going through the initial setup with the replacement box.