Many thanks to all of the Loungers who offered thoughts and advice - much appreciated.I finally gave up and took the laptop back to the shop where I bought it about 3 months previously. They tested it for about 1/2 hour and then sent it off to Acer for repair. A week later they called us to go back to the shop and collect a brand new Acer replacement. Thanks to Chi Chang computer store and Acer in Chiangmai, Thailand for the great service - it's one reason why Acer is the top selling brand here.