Tv keeps switching off every 40 minutes or so.Switch off and on on tv and it comes back on again? Any clues.

Could just be a cold/bad solder connection on a cicuit board, a cap is bad, or any number of transistors or ICs breaking down when hot. Make sure the sleep/timer is not on. The best thing to do is take it to a repair shop where they can watch voltages and wave forms to see what shuts down. They can also freeze parts to see which one may be causing the problem.