Hi. I have just bought the Lenovo Flex 15. I looked at the power settings and set the brightness, but it is ignoring the settings and always either darker or brighter than I want it. It keeps flickering between being darker and brighter and it's giving me a headache. It is unplugged if that makes a difference, I haven't used it plugged in yet to know if it does it then. How can I stop it? I've looked through the settings and can't find anything. Thanks.