Panasonic Viera screen flicker cycle

Panasonic TV TX-L42E5B has developed a screen flicker. No apparent obvious cause, it started while watching.

Flicker appears as blurred picture, horizontal lines scrolling up. Similar to losing the V-hold on an old CRT.

Flicker appears with no signal input so fairly confident it is internal to TV. Have tried with alternate power supply, no change. No other equipment appears to be affected.

Flicker appears for about 2 seconds, every two seconds (i.e. 2s on, 2s off).

Any solutions of a home-quick-fix?