Rear Projection tv fades away PT-51G45CU

I had a large projection tv given to me and when you first plug it in the picture is there but after a minute or so it just fades away and you get a blueish grey bright screen but sound is always fine. If you unplug it and plug it right back in the picture is back but does the same thing. It's a panasonic model PT-51G45Cu, chasis AMEDP809 made around 1996. Anyone got suggestions?