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Thanks Agent WD40. It WAS the monitor after all. I took the whole lot into the local computer shop, and they tested the computer and found there was no problem. They wouldn't look at the monitor though. They said that, basically, it was a TV, and the voltage was too high for them to work with, so it would have to go to a TV repair shop. Now, knowing they charge an arm and a leg just to talk to them, I cut my losses and got a nice new monitor!! Hope you manage to get yours sorted!