When using lassoo or patch tool it seems like a part of the screen goes black / as if lagging.....

Good afternoon.  Please help me with an issue I am having for the first time today while editing newborn images.  When using the patch tool or the lassoo tool the screen seems to make a black block over the image.  Almost as if it moves one part of the image and leaves the black background behind.  I have to click on the image numerous times for the screen to return to normal again.  I have 20 gig ram and my memory isnt even a quater used.