I had the same problem with this same model. Sadly, it seems to be a common issue after about 2 - 3 years of use. I called Samsung & they gave me the name of a guy in my area. He came to my house & fixed it for $150. I watched him fix it and asked questions while he did it. It's your power supply. You need 2 new capacitors. They are blue round tubes. You'll know which ones they are because the tops will be buldging out & not flat like all the other capacitors on the board. He didn't feel like soldering new capacitors on my board so he just installed a whole new board & took mine in trade. It took him less than 15 minutes.
I am definately not a "QUALIFIED" electronics guy or have any soldering skills. However, I can tell you that after watching this guy anyone could have done this if they just want to switch out the whole board. All you need is a blanket and a long handle screw driver and a new board. Lay a blanket on the floor, UNPLUG THE TV and CAREFULLY lay it on the blanket face down. Unscrew the back, pry off the plastic and disconnect/unplug the computer board in the center. (No tools needed to unplug the board. It's the only thing in the middle & really obvious to see and unplug!)
Pay attention to how you unconnect it so you can plug it back up easily. Although, it seemed like each connection could only go into it's assigned slot. (i.e. - Tab A to Slot A)
Put the new board in that spot, plug the wires back in (it's like only 4 plugs) Put the back on and screw it in. You can turn it on & everything should be ok, but the next step is important!
FINAL STEP - After verifing that it works again. Leave it UNPLUGGED for at least 4 hours. (Unplug it from the wall - not just turned off) I recommend doing this overnight or up to 24 hours. The internal computer needs to reset itself. If you start using it again right after the repair the screen will go black after a few minutes of continuous use until you allow the computer to reset.
Again, I'm not a "qualified" electrician and you should make sure to take all saftey meaures (Don't leave it plugged in while you fix it!!)
However, if you don't feel you can do this call Samsung (1-800-SAMSUNG) and they will give you the phone number of a qualifed guy near you. The capacitors only cost about $20, if they just solder new ones on the board. The rest of the bill is just their boated labor cost for a 20 minute fix. (Get a quote up front on labor cost for this easy fix. Use this information to sound like you know your stuff, so they don't try & overcharge you on labor) Do not pay more than $200 for this if they come & do an in-home repair.
If you want to just order a new power supply board & do it yourself the EZ way I wrote about, the board will cost about $100. Just google the model # and you'll find part stores all over the net.