I have a Panasonic PT-53WX52F rear projection TV that has worked well for past 5 years. Recently it would turn off after anywhere from 5 - 30 minutes. You then have to unplug it from outlet before you can turn it back on. It may turn off again - or maybe not. No pattern on how long it stays on. Picture looks fine. Interestingly enough - it only turns off when watching TV. Will stay on indefinitely if we're playing a DVD. No extension cord in use. Any clues???