A couple of weeks ago I couldn't get my Tab S to start. A forum member instructed me to reboot by holding down the volume and power keys for 10 seconds. It worked and the system booted back up. Ever since, the Tab S has been unstable. The screen periodically jumps or wobbles and sometimes goes dark for a second or two. In addition, a popup continually appears asking if I want to shut down, restart or activate sleep mode. Can anyone offer a suggested fix or is my machine of 1 1/2 years about to go belly up on me.