Intermittent failure to fully boot in fresh 16.04

After using 14.04 for a long time, I did a fresh install of 16.04. I realized my SSD was not ruing the TRIM function but is set now to do so in 16.04. I had no issues with install. After changing the desktop background I noticed that sometimes after logging in, I would not get to the desktop. I would power down and then it would work. It was begiing to look like 'every other time' that it would not work. I also used ctrl-alt-f1 and typed REBOOT at terminal command and that seemed to work too. i went back to default background and it booted up fine this moing. Is this a Ubuntu bug, a memory bug, a video driver bug, etc? 14.04 ran fine on this laptop so not sure why 16.04 would have issues with default settings. That was my only thought about going back to original background. Otherwise, once I am up and ruing, all runs well and fast!