Yosemite upgrade–continuous slow screen redraw
I had this problem and I solved it by resetting the PRAM/NVRAM, which is done via:


Shut down your Mac.
Locate the following keys on the keyboard: Command (⌘), Option, P, and R. 
Turn on your Mac.
Press and hold the Command-Option-P-R keys immediately after you hear the startup sound.
Hold these keys until the computer restarts and you hear the startup sound for a second time.
Release the keys.