Samsung HDTV Oval rainbow of colors covering entire screen

I have model P2764 for 2 1/2 yrs now. A few months ago my cable co had to add a booster and do some additional wiring because my house is 400+ feet from the road and they needed to do when I changed to a cable bundle package. A week after this happened, I was watching TV when the power went off for a second or two. When I turned the TV back on, there was an oval rainbow of different colors covering the entire screen and straight lines were on an angle. I called Samsung/Cable Co./Power Co./Retailer and it boiled down to setting up an appt. for a Samsung dealer to come out and service it. 48 hours after it happened, it suddenly was fine when I turned the set on, so I cancelled the appt. About a week ago, I mistakenly unplugged the tv and when I plugged it back in and turned it on, the oblong rainbow is back. I unplugged it immediately for about 12 hours but it still has the problem. Any suggestions would be appreciated. Thank you in advance