First try a hard reset . Unplug the tv from wall plug and hold the power button on tv for 60 seconds.Let it sit for 10 minutes.Plug in turn on.
If that doesn't work it's most likely a dry solder joint , or a component which has become thermo sensitive (i.e. its value drifts with warmup), or an capacitor which has gone out of spec. All common in TVs.
flyatyou66, August 2014