Pictures are important. When you said "white lines on half the picture" I imagined them being horizontal, on the top half of the picture, not vertical, on the left half. These lines are not VBI interference. They are horizontal drive lines.
http://www.audiokarma.org/forums/showthread.php?t=68042
They're not Barkhausen lines (so don't go down that path).
Ringing in the horizontal oscillator. Most vintage TV repair books have a chapter on it. Yout HO tube might test good but still have a problem, try a swap. Of course, always suspect wax/paper caps in the area.