Blank Screen of death, once in a while

It first happened a couple of weeks ago. While working, I get a blank screen. No more mouse. Nothing happens when I move the mouse or enter a key.

I don't remember exactly what happened, but here is a rough recollection of what happened:

I usually use an external HD monitor with the Mac lid closed. So, I open the Mac lid and hold power button down until Mac turns off. I press the power button. Mac starts up with logo. The progress bar progresses a little. Then it shows a blank screen. I power off and power on. Then it shows "Your system restarted...press any key or wait a few seconds..." (I don't remember exactly this message.) I press a key and the boot up resumes. Then it shows a blank screen. I power off and power on. Then I get a blank screen. I power off and power on. Then it boots up correctly.

The Mac worked fine until yesterday. While working, I get a blank screen (I think it was gray). No more mouse. Nothing happens when I move the mouse or enter a key. After a while the fan speeds up to high speed. I power off and power on. Mac starts up with logo. Then it shows a blank screen. I power off and power on. Then it shows a blank screen. After a while, the fan will speed up. I power off and power on. Then I get a blank screen. I power off and power on. Then it boots up correctly.

So far, my Mac is still working. What is causing this? How do I fix it?

I switched over from Firefox to Safari 9.1.1 about a month ago. Is Safari doing this?

My system:

MacBook Pro (15-inch, Early 2011)

OS X 10.10.5

Processor: 2.2 GBz Intel Core i7

Memory: 8 GB 1333 MHz DDR3

Graphics: AMD Radeon HD 6750M 1024 MB

512 GB SSD