32 " Wharfedale flatscreen TV. After 2-3 hours it switches off?

After 2-3 hours it switches off. Then switches off every 20 mins or so. It can be started again by pushing the start button on the remote control. Sometimes after several goes the picture goes but the sound remains. In this instance the TV has to be switched off at the plug to start it again????[The TV is 9 moths old ex demo off ebay no warranty left] Owen