Anyone know what causes this?

[Einar72]

Looks like Barkhausen interference (aka snippets) to me. My 1970's tech-school copy of Grob's Basic Television, covered the subject adequately (but not in the 1949 edition, though). In a nutshell, these undesired oscillations from the horizontal output tube show up as vertical lines down the left side of the raster. They are usually black. Keeping unshielded antenna lines away from the horizontal output section also helps.