Unfortunately Samsung had bad panels in these models that had that exact symptom. The fix was to replace the panel. They also had power supply issues. I would check the power supply for swollen capacitors. I will bet that there are some bad ones on yours if it is over 15 months old.
Check: CM802, CM803, CM807, CM808. Replace these with 35V 1000uf.
Check: CS804, CM812, CM813. Replace these with 16v 1000uf.
Replacing the capacitors may or may not fix the set but the power supply has to run correctly before anything else will and certainly before further troubleshooting.