I've just come on here to post about this problem. The last couple of months the picture has been pretty ropey for the most part. Its quite annoying!
Just tried the link on this thread to test things and that flags no problems.
Can it be fixed by replacing with a new box?
If this is the case on all channels, then it's a signal issue, which will need a tech visit. If it's only present on recording then it's a dying HDD within the box, which you may be able to swap yourself, but likely also will need a tech visit.