Disappearing Brush Size!

Anyone have any suggestions why the brush size (the circle around the cursor indicating the brush size) would disappear?
I've thought it might be some kind of memory (RAM or DDR) issue, but if I open or make a new image file the size shape is there, and if I go back to the other file it's not.
Now, don't go suggesting that perhaps the brush is too big for the view. I've thought of that and it does not show from size 1 to 1000. It is not the color of the image. I have experimented with various colors in the background. The same image copied and pasted in a new document shows the brush fine.
Now, the real kicker. After a while, SOMETIMES (see this is not easily repeatable, so I was thinking memory) even the new document will not display the brush size.
Shutting down PS sometimes works. Completely rebooting seems to be the only cure. Again, memory, to me.
I have run diagnosis on my RAM and video memory and all check out o.k.
Also, these are not large images I'm working with. Sometimes big, sometimes small. Makes no apparent difference.
Any ideas?