Had the same issue after heavy CAD use of my iMac (same machine, late 2012 3.4 GHz) for some hours, and it continued to happen. After I found this thread, I decided to check the outlet and turns out it was faulty, grounding wire was missing on the back side! Plugged the iMac into a good grounded outlet and it’s been running fine since.

The 6900M GT is actually smaller than the 9400M.

So wtf gets my correction of that mistake denied for the third time now?

Look at the pictures, dude! The larger chip is the 9400M.