Does the TV work correctly without the set top box attached?
Does the TV function correctly when viewing TV directly through the set top box?
If the answer to both the above is 'yes' then the set top box is faulty in the playback mode. This could be (and from your report on symptoms, is) an intermittent type fault possibly resulting from such as a 'dry joint' or failing component.
It is possible the programmes recorded perfectly but the playback circuitry in the unit has a problem.
If it is still under warranty you should return it for repair and ask them not to wipe the HD.