I found the problem of varying screen brightness
This is actually my first post as I've been wandering around xda for a while as a guest and enjoying the hi tech professionalism here. When I saw this thread I HAD to join....
I have a new 10.1 2014 edition LTE T-Mobile version that T-Mobile unlocked for me this week and I am using it on AT&T LTE and it works great!! I have not rooted this device, it is completely stock.
I installed a few different keyboards to try out which were Swiftkey, Google Keyboard, TouchPal X, Swype, and the stock Samsung default keyboard.
I noticed last night in my living room while typing an email that the entire display was faintly pulsing bright/dim in perfect timing with the blinking cursor. After much trial I found out it only happens when using Google Keyboard or TouchPal X. If I use Swiftkey, Samsung, or Swype keyboards the screen does not pulse with the blinking cursor. So needless to say I uninstalled Google Keyboard and TouchPal X. They must be extremely heavy apps in terms of RAM usage maybe?? This is where you guys can take over....