Hi hornplayer,
Welcome to Sony Community!
May I know what is the model name of the computer? Let me verify, when you turned the monitor off then turned it back on, did it fix it? How often does this happen? If it happens frequently then i suggest that you buy a new monitor since you mentioned that it's already 10 yrs old. It's probably needs to be replaced. You also need to check the over all performance of the computer.
Regards,
Jen
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