Hi Eric,
They haven't fitted a new PSU yet, but they are due to send somebody to do so on Monday. Dealing direct with Samsung has been a nightmare - I contacted them both last week and earlier this week about the problem, was told both times that an engineer would call me back within 2 days to talk about the problem - only for them to completely forget to do so. It was only me being a bit shirty on the phone this morning that seemed to get them moving, although the service centre whose number they gave me were ten times more helpful.
Not sure why your screens would revert back to the desktop - even if they're turning themselves off, it's likely that it would go back to displaying the MagicInfo software.