This 2010 MBA has El Capitan 10.11.4 installed. The trackpad has recently begun to behave unpredictably, selecting text or a new tab in the browser or another open application, even when I have not clicked Select. I have tried to boot the MBA in Diagnostics, but it simply booted in Ordinary mode. I tried to run Apple Hardware Test online, only to see a message saying that it was not available for this notebook. Where do I go from here ?