Testing mode? is that when it asks if everyting is ok? and has a yes and no at the bottom? if that is what you are talking about, I see it once in a while but I think my wife pushes the help button or something because "she needs help" haha. Anyway, I believe you can turn that off in the set up. If thats not it then yes I would return it. Mine never restarts or shuts down unless the battery is dead.
As far as what you were talking about with covering the sensor when switching between activities and a single device, yea, thats what I do too. I havent seen a way around it yet. All and all I have no complaints.