Why does my Magnavox Projection TV screen flash off and on?

Have a Magnavox 51" Projection TV while watching it the power stays on and the screen will go blank and flash the picture on and off and back on.Only seems to do it after I have been watching awhile.The power remains on the whole time.Sometimes the pic will get lines in it for a few seconds when it flashes.Sometimes the pic will stay blank for about 20-30 seconds and then come right back on?Any help is appreciated.