JVC television has black square in middle of television on most channels

Hi, I have a JVC AV 27710 and it was purchased in 1997, it has recently started to put a large black square on the television screen after about two seconds of being on a channel. I have a couple channels that no longer do this after fiddling around with the menu, but cannot seem to replicate this across all channels. Can someone suggest another thing to do???

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