Questions to ask:
Is this pattern visible on one station or all stations?
Does the pattern crawl up or down the screen?
Is anyone else in the area having similar problems?
Can you change the pattern by bumping the set? On a healthy set you should be able to wack it with the flat of your hand without any visual or audible problems. (As long as you don't cause any physical damage to the cabinet, controls, or your hand, hit it as hard as you can. Sets are not particularly fragile.) Be sure to hit in all directions.
Is the set sensitive to jiggling any of the connecting wires?
If you have other TV sets in the area, do they have any similar problems?
Are you keeping a written log of problems? If so, can you correlate TV problems with anything? You mentioned time of day. What about weather, day of week, other appliances in the area, computer use, video games, power tools. Look outside; what is happening out there?
Did anything in the area change at about the same time as the problem started? (construction, new neighbor, utility crew working in the area)
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