Why does my Screen Shift?

At first, I say that my iMac has a Solid-State Drive, or SSD. This makes my iMac act speedily, or like-new! :-)

But sometimes, the whole screen of my iMac shifts leftward about one-quarter of an inch. My iMac then freezes for 2-seconds, not echoing what I type, nor responding to my mouse, neither.

This freeze lasts less-than 5-seconds, but it interrupts my oh-so Precious train of thought as I type.

[About the capital P in Precious, have you read “The Lord of the Rings” trilogy by J. R. R. Tolkien from the 1930s? If you have, then you know what I mean by Precious. If you haven’t read Tolkien’s trilogy, then just let my comment Pass you-by. :-) ]

Do you know what I can do about this brief freezing of my screen? Anything? Thank you.

P. S. By the-way should it matter, I Have & I Use TechTool Pro, v9.6.1, by Micromat, Inc. It is not as useful with a Solid-State Drive as an older Hard Drive, but TechTool Pro _can_ find corruption. :-)