I hate to say it, but that MB Pro's ATI graphics card may just be finally failing. Most gave up long ago.
I have the same model that was repaired on Apple's extended repair program, long expired, to address the display problem with those models. But the MBPro wouldn't even boot, just dead with a black screen.
I hope not, but just a forewarning in case software fixes don't work. ;(
And have you tried booting up into Safe Boot Mode and see if things improve?