CRT image is contracting/expanding quickly?

A couple days ago the image on my CRT has started acting strange, the image on the left and right will contract a few pixels, and then expand back to where they should be within less than a second, making the image look like it's twitching or something. The top and bottom of the image stay where they should and do not have any problems.

It will get faster/more extreme if I watch tv for a while, but turning the tv off and then back on will return it to normal for a bit.

The problem is most noticable when I pause the video, as the still image will twitch like that. The outside edges of the picture do it the most, the middle of the screen basically stays still.

What might be causing this? I have repaired consoles and done a bit of soldering before, and am aware of the risks involved but I really like my TV and want to fix it.