Hello, and sorry you're having problems. It appears Rusty has opted out but I will be glad to assist you.
If the issue happens on all sources, it's generally caused by a failing backlight circuit in the power supply board, which typically starts out as intermittent and gets worse until the backlights fail. This is a replaceable component in the set however, and if you perform the work yourself the part is around $100 (give or take).
That being said, just to make sure this is the issue, please try this test:
When the image is flickering grab the original Samsung remote and press MENU- we need to know if the TV's native MENU also flickers. If so, then this is the supply/inverter board- which is an average of $250 parts and labor at a shop (or a $100 board if you want to swap it yourself).
However if the MENU does NOT flicker- even when the picture behind it does, then you're looking at a bad Mainboard- about the same cost as the power supply/inverter.
Let me know, and also let me know if you're looking to repair this yourself or not and we can prcoeed.
Feel free to reply back if you need more help, or want to reply back with more details. It does not cost you anything more to reply back once you've rated. Please do rate me positively (at least 3 STARS) and I will continue with you until this is sorted. The positive rating ensures I am credited for my work.