I was having this issue & it was due to the horizontal setting being slightly off to the left. The image is being bounced off of the side of the tube & causing weird video artifacting on the left side, especially with the test cart's test pattern #3, if you have that & know what I am talking about. If not, no matter! This is just something you can try.
Here's what fixed (most of it):
See the attached diagram of the power board. You do not have to dismount it. There is a hole from its underbelly that allows you to insert a screwdriver. Do so & slowly turn it one way or the other to horizontally shift the image.
I am still having a bit of an artifacting issue but that fixed a lot of it. Hope that helps!