Repairing LeCroy 9400A, possible memory error

[kubark42]

I have a LeCroy 9400A which has problems with the first channel. It seems to read data okay, but there are these weird marching lines.

https://imgur.com/a/V5wEOLk

They seem to be the same height as the observed data, which makes me think that this is some kind of display or memory issue.

Anyone seen something like this? Thoughts for where to start?