Sony 57WS500 horizontal white lines with good picture

Hi,

 I just installed a HDTV converter box set at 1080i. When the screen is dark or black I can see horizontal white lines that dip at the ends of the screen. The shape of a frown. If I change the box setting to 480i, then there are less lines but they are still there.

The white lines look like it is a problem for the flyback circuit. Is this a horizontal or vertical flyback problem? Are there any parts I could check that are likely to cause this problem? I just installed new Sanyo STK392-570 ICs in the TV a few weeks ago to fix the convergence problem. The picture looks fine, but the horizontal white lines are seen when the screen is a dark color. The horizontal white lines are not perfectly flat. Maybe at 5 degrees or so from horizontal.

Sony

Model: 57WS500 

 Thanks