Desktop screen on macbook pro flashing black, dock missing, view of windows messed up
Got mac pro fairly recently w no problems till now. After turning it on out of sleep desktop started flashing black non stop. Tried several times shutting down and restarting but would just happen again. Application windows work fine but won't enter full screen mode (I get the "cant do that" beep when i try). Desktop behind windows keeps flashing, sometimes "splits" screen covering portion of windows. On top of flashing its like boundary bw screens is messed up, sometimes split between two or cut off. Also the dock is missing. Help!
Mac Pro, OS X El Capitan (10.11.3)