Denying a product is faulty, and thanking us for patience is a bad sales strategy

We bought a Mecer 55S92U 55" UHD LED Smart Panel Monitor almost a year ago now, which randomly switched itself off whenever it was used for netflix or high definintion movies from ext hard drive. we reported to them after a few months of use. It was returned, sent back, returned, sent back, returned and sent back. They kept saying it was us., but it would only happen to this screen, not others, and it was not a setting. After returning it for 3rd time, a technician called to say it was a factory setting to go off automatically after 4hours of use. (But it would go off after a few minutes of use. ) And the screen when switched off, and back on again was all green and pixellated and stripey . After 100s of emails back n forth, all very polite from them, and increasingly irritated from us, we have achieved nothing. They still deny it is faulty.