Periodic blank desktop

I am having an issue with my rooted, unlocked phone on 5.1.1. Every once in a while when in an app and pressing the home button to go back to the desktop I will instead get a blank screen. The navigation buttens are visible and respond when pressed (a gray background shows briefly when pressed) but don't do anything. I can pull down the notification shade but nothing happens when I press a notification. I can press the setting button but again nothing happens. This hppened once while I was playing music and the music continued to play. I have to turn the phone off and then back on again to get it to work again. I thought it might be a Nova launcher problem so I went back to the default Google launcher but it still happens. As a last resort I reloaded the factory image, re-rooted and cleared all ther data to start from scratch but it is still happening. Has anyone seen this before or have any ideas what is causing this? It is driving me crazy.

Thanks,
John