Graphical artifacts on MBP 2012 13" requiring shutdown - help!

Hi,

I'm having the following graphical errors on my MBP 2012 13". Can you please give me your assessment?

Specs:

• MacBook Pro (13-inch, Mid 2012)
• OS X El Capitan 10.11.4
• 2.5 Ghz Core i5
• Intel HD Graphics 4000 1536 MB
• 8 GB 1333 MHz RAM

  It has 8GB aftermarket RAM at 1333 MHz instead of 1600 MHz (supported) -- was told this is OK and had no problems with it or the SSD for a few months after the upgrade. This graphical problem is new and I don't think it's related.

• 512 GB Samsung 850 Evo SSD (aftermarket)

Sometimes I also experience massive choppiness and lag in CS: Source, even though my ping is often low (sub-100, sub-50, or sub-30). Not sure if this is related.

Note: This isn't dependent on being plugged into the monitor. In the past few days, it has happened 3 times, 2 of which were on another monitor and in a different user account.

Whenever this happens, I have to press the power button to restart. It seems that the computer gets very hot too.

Please help! What am I dealing with here?