When swiched on, the screen displays green vertical bands which almost obscure the picture. By switching off and then back on, the green bands disappear but the vertical edges on the image are very unstable. After about 10 minutes, the picture stabilises and is fine until the next time it is switched on from cold. Any ideas?

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Most likely a power supply component has aged and thus changed value and/or failed. Take it to a reputable repair person.