Mac sometimes freezes when waking up from sleep

My Mid-2012 MacBook Pro sometimes freezes when I wake it up from sleep. It shows the lock screen, but before I'm able to enter my password, it freezes. I am left with no option but to press and hold the power button to force it to shut down.

I checked the Console log and saw a long of list of messages saying the same thing (at the time the computer froze):

WindowServer[663]: GPU Driver appears to be hung (over 5 continuous seconds of unreadiness)

The computer runs fine otherwise. This problem occurs only when waking up from sleep, and that too rarely. I haven't been able to find a pattern. The problem seems to have started after upgrading to OS X El Capitan, but I don't know if it's related.

Also, it passed the extended Apple Hardware Test.

Can anyone help, please?