Using Mini DisplayPort to HDMI cable

Using a Mini DisplayPort to HDMI Cable from a 2010 MacBook Air is playing up, making it practically unusable. Essentially what happens is that when connecting a television set using a Mini DisplayPort to HDMI Cable the picture is perfect but no longer than 30 seconds at a time before the picture 'goes to sleep'. Is there a setting I need to change? There must be a setting that I can change which I haven't worked out yet. Thanks in advance for your help