2011 Macbook Pro problems out of nowhere

I'm running out of ideas and patience to be quite honest!

Out of nowhere my macbook started acting up and I'v began experiencing problems I've never had before.
I have a friend who used to work at Apple and he's talked me through all of the usual stuff you'd be prompted to try/have tinkered with at a genius appointment.
I'm experiencing glitching/artefacts on my screen, predominantly when watching videos on YouTube... It doesn't seem to be an issue when watching video files on my system. This usually leads to my macbook jumping or freezing for 20 seconds before turning itself off or restarting.

Upon restart, my macbook fails to reboot successfully without the aid of key prompts like booting in safe mode or resetting the SMC.
I've recently upgraded to El Capitan, however, the problems started the night before I did that. I only installed the newest OS because I thought it might fix my problems but alas, it hasn't... I now just have a macbook that crashes with a slightly prettier interface.

What I'm running:

MacBook Pro (15-inch, Early 2011)

OS X El Capitan

Processor: 2GHz Intel Core i7

Memory: 8 GB 1333 MHz DDR3
Graphics: Intel HD Graphics 3000 512MB

I'm running it pretty vanilla, the only thing that isn't factory default is the RAM which I upgraded to 8 GB itself (this was done over 2 years ago so its unlikely this hardware upgrade is at fault for what I'm experiencing.)

Any help would be much appreciated, I'm reluctant to take the machine in-store until I've exhausted all options.

I don't know if it's worth mentioning but I typically only use this machine for Pro Tools work, basic internet browsing and music streaming.