Display showing grid lines and craziness.

How can I find out what is causing this issue?

From what I've read it could possibly be a failing graphics card, failing LCD, or loose wire.

Here are my specs:

iMac (27-inch, Late 212)
Processor: 3.4 GHZ Intel Core i7

Memory: 32 GB 1600 MHz DDR3 (upgraded myself)
Startup Disk: Macintosh HD

Graphics: NVIDIA GeForce GTX 680MX 2048 MB

I'm a photographer and videographer, so I'm sure my graphics card gets quite a workout often. I've also come back to find that my computer has restarted itself, likely due to the issue.