I have faced a similar problem on my 465B. In my case, horizontal sweep used to shrink and expand at random. There was no linearity issue, as calibrator square wave looked uniform across the width of display, only shrunk randomly to anywhere from six to ten horizontal divisions. Fault was traced to an intermittent x1 gain setting potentiometer in the horizontal amplifier. This pot was replaced and gain adjusted as per service procedure, and scope has worked fine since then.
In your case, it may be a noisy horizontal position potentiometer, in addition to x1 gain potentiometer. Since you have linearity problems also, as you see shrinking of square waves at some positions of screen, there could be linearity problems in sweep area, or some limiting due to a failing component in one half of the horizontal amplifier section.
My two cents to share, hoping you can get some ideas to explore.
Shailendra
