I came across this peculiar OS X bug today. I don't know what causes it, but I do know how to fix it.
If your computer is flashing alternately between your desktop background picture and solid grey colour, and you have no Dock on the screen, then this should help you out:
- Start your computer whilst holding the Shift key. This will perform a safe boot.
- Click on the affected user account and type in your password, but DON'T press Enter.
- Instead, hold down the Shift key and click on the login arrow.
This should get you back up and running.
The problem seems to have something to do with the Dock continually crashing and invoking the CrashReport app, which in turn slows the machine down to a snail's pace. I'm not sure why the background also flashes grey.