Lost side, upper, bottom bars periodically in LR, graphics card?

No, I've checked all the parameters, the "arrows", etc. but every now and then, LR just blinks and then only shows the center screen of whatever I'm in..library, develop, etc.  I have to exit LR and then it comes back OK...but will reappear again another day. Any clues?

(Using Mac 10.11.4, Imac 27 mid 2011, 2.7 i5, 32 GB 1333 DDR3, AMD Radeon HD6770M 512 MB)

I'm thinking that it might be the graphics card?  (This used to be my husband's computer, but he's doing 4k video, so he got the "new machine.") Perhaps getting overloaded? I use a lot of PP programs with my photos.