Picture was dark, slow to develop across the screen from right to left; tapping, pushing or squeezing the upper left corner of the screen would sometimes restore normal brightness and picture. (Two separate problems)There was also a wide vertical band of lines on the right hand side of the screen and sometimes on the left.
The T-Con (Timing and Control board) located at the top middle of the back has (4) flat ribbon cables that can develop bad connections on the contacts. Some, usually only one or two will be the problem.
Gently remove the flat RF aluminum shield at the top frame edge to get at the ribbon cables. Pull down the small brown locking tab to free the cables. Then with slight pressure on the locking tab, gently move the ribbon cable up and down into the connector to restore proper connectivity. You may have to do this several times.
Note: your screen many have the band of lines at the far left, middle or mid left or mid right on the screen. As viewed from the front, the left ribbon cable will cause lines on the left of the screen and the far right ribbon cable will cause lines on the far right of the screen.
The above picture shows the left connector that was the cause of the band of lines on the right side of the screen in my TV.
Some message boards also recommend putting a thin piece of tape across the back of the connector, opposite the connection side to put more pressure on the conducting pins for better contact. It should be thin to avoid putting undo stress on the locking tab. If you break it you'll have to buy another T-Con board.
To fix the intermittent screen and lines on screen located in the upper left of screen, the aluminum frame surrounding the screen will have to be loosened, not removed. All of the small black screw at the top and sides will need to removed and some of the bottom ones to. The frame is flexible enough that complete removal is not necessary. Gently pull the frame away from the top of the screen - only a couple of inches is necessary. At the top left is a small ribbon cable that is the cause of the flashing lines and dark picture. I used a small, round .25"/6.5mm piece of foam and placed it across the edge of the ribbon cable while pushing the frame back into place. See picture where I placed mine to fix. Only use enough length approximately 2" placed over the ribbon cable, not shown, behind the frame as I'm holding in the pix.
These fixes apply to multiple models and screen sizes of flat screens. It should be noted, that if the band of lines on the screen remains stable no matter how much you move or try to clean the contacts on the T-Con ribbon cables then replacement of the T-Con board may be indicated. Good luck.