Hi. I'm Steve. I'm sorry to hear about your TV. I will try my best to give you a precise and accurate answer.
How long are the intervals between turning on and off and on again?
it stays on for 20 secs than shuts off and turns on
The most likely culprit is a failing power supply. Typically, the board can't sustain the voltages needed and the TV shuts down. When the load is taken off teh supply (by shutting down) the voltages come back up and the TV comes on again - and this cycles as you are experiencing.
The fix would be to have someon look at the capacitors and other components on the power supply board or to simply swap out the board.
Here is one for $66US http://www.shopjimmy.com/sanyo-1av4u20c17401-power-supply.htm
If you can't swap it out yourself, add about $150US to have someone do it. It's not hard if you know how to use a screwdriver - simply remove the back of the TV and the PS board will be right there. Swap it out. Plug and Play.
Just ask if you need more help on this issue. I will be here.
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Thanks very much.