I pulled this from theandroidguy.com
GALAXY S5 SCREEN FLICKERS WHEN BRIGHTNESS IS SET AT CERTAIN LEVEL
Problem: When the screen brightness is lowered to the lowest setting, the lower half of the screen begins to flicker. Raising the brightness a few notches stops the flickering. I have not dropped the phone or exposed it to water – the issue started out of the blue and it’s driving me mad! Please help.
Troubleshooting: Screen flickering issue can be hardware or firmware problem and since there’s not much information here, you have to other option but to rule out the possibility that it’s just a firmware issue (which I really think it is) by performing hard reset. Of course, backup all your important data before doing that as they will be deleted.
Turn off the device.
Press and hold the following three buttons at the same time: Volume Up key, Home key, and Power key.
When the phone vibrates, release the Power key but continue to press and hold the Volume Up key and the Home key.
When the Android System Recovery screen appears, release the Volume Up and Home keys.
Press the Volume down key several times key to highlight ‘wipe data / factory reset.’
Press Power button to select.
Press the Volume down key until ‘Yes — delete all user data’ is highlighted.
Press Power button to select and start the master reset.
When the master reset is complete, ‘Reboot system now’ is highlighted.
Press the Power key to restart the device.
If the hard reset failed, then have a technician check the problem for you.