I am running a mid-2011 MBA with 1.8 Ghz Core i7, 4Gb RAM and a 256 Gb SSD. The lat couple weeks it's been locking up on a regular basis. I'm not doing a lot on it. Things that cause it to lock up include: downloading large files on iTunes, running a scan with Tech Tool 6, running Sophos virus scan, deleting large files from trash, deleting free space, and other tasks that would take several hours. The MBA seems to lock up if the screen saver activates. It runs fine when the screen saver is not running, but locks up and requires award restart wy too often. Any recommendations? I am running all up to date software.