My tv keeps going dim and then black but the sound keeps playing

So I have a Samsung TV. I don't know exactly how old it is but its atleast 5 years old. it also has roku. it keeps going dim and then going completley black but the sound keeps playing and if i press back the sound stops playing so i'm assuming that its still working. I'll turn the TV off and then back on and it'll be fine for a few minutes until it does it again. everything seems to be plugged in. What could be the problem? is it just broken?