When the power is turned on the image appears on the screen, then after about 1/2 second the screen image disappears. The power light remains on. If I force a change of display mode (from my computer) the image returns and disappears again. (MAGNAVOX 15MF400T/37) There is no difference if I have it in TV tuner mode, PC mode, or A/V mode. How can I troubleshoot or repair this TV/Monitor?