My screen shifts Left and Freezes. Why?

My iMac uses a _fast_ solid-state drive. All is Good, but I see a _wee_ problem. Please let me explain, OK?

Sometimes -- I don't know Why -- the whole screen Shifts leftward. Over about 2-seconds, my iMac fails to echo what I type. [I _think_ that what I type appears eventually, so it is buffered. :-) ]

But this delay, as brief-as it is, Crimps my writing. My query to you is, "Can I do anything about this freezing?" Need I get used to-it? Thank you.