Another kind person (not me) answered this same question very recently - and offered several links.
If you've got a scrambled picture it means the digital file is too corrupted for the electronics to cope with. That could be caused by a weak/corrupted signal being received - a matter of poor reception - or by a good signal on the disk not being read correctly - I assume the programme is stored on a disk - the file on the disk either being written to disk - or read from disk in a corrupted way.
My guess is that the disk in your machine is not working as well as it should - what is significant to me that you report a particular programme being OK on first viewing - indicating that it was received and written to disk OK but scrambled on a subsequent viewing. i.e. Disk read errors.
However - it may be that the electronics that work out if the file is corrupt or not could be playing up.
Disk read errors or file integrity electronics playing up.
I know nothing of these machines - I just guess.
There will be no reset button as far as I know.
My expectation is that this problem will not go away and will probably get worse over time.
Good Luck...
April 2016
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Try fixfault.com to fix dtt receivers.