My sansui sv2118i has 5 or six visible flyback lines across the centre of the screen. ?

My sansui model sv2118i has a perfect picture,except for a 5 or 6 flyback lines across the centre of the screen. This occurs om all channels. The picture is about 2 inches away from the bottom and the line is worse at switch on.the picture size comes perfect when the set warms up but the line stays but gets better.