NotTaR of Television Sets : Intermittent jumping or jittering of pic..  
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Intermittent jumping or jittering of picture or other random behavior

This has all the classic symptoms of a loose connection internal to the TV or monitor - probably where the deflection yoke plugs into the main PCB or at the base of the flyback transformer. TVs and monitors are notorious for both poor quality soldering and bad connections near high wattage components which just develop over time from temperature cycling. The problem may happen any time or more when cold or hot.

The following is not very scientific, but it works: Have you tried whacking the TV when this happened and did it have any effect? If yes, this would be further confirmation of loose connections.

What you need to do is examine the solder connections on the PCBs in the monitor, particularly in the area of the deflection circuits and power supply. Look for hairline cracks between the solder and the component pins - mostly the fat pins of transformers, connectors, and high wattage resistors. Any that are found will need to be reflowed with a medium wattage (like 40W) or temperature controlled soldering iron.

It could also be a component momentarily breaking down in the power supply or deflection circuits.

One other possibility is that there is arcing or corona as a result of humid weather. This could trigger the power supply to shut down perhaps with a squeak, but there would probably be additional symptoms including possibly partial loss of brightness or focus before it shut down. You may also hear a sizzling sound accompanied by noise or snow in the picture, static in the sounds, and/or a smell of ozone.