How Can I fix my Ascent Television ModelAFTV2702-Picture?

Our picture keeps going off and coming on. You'll be watching TV and sll of a sudden the picture goes to black with a colored vertical line in the middle of the screen then totally black. Will be black for 20 minutes or so, and then the picture comes on for 2, 5, 10 minutes, then black again.
We have sound the whole time.