While playing the Sims 4, my PC will suddenly shut off.

I have yet to test this but I think it could be right, I have also experienced this with Rebel Galaxy (really bad with that game as it corrupts the save forcing you to restart)

I have been dropping a few times since the recent Origin update, ie a couple of days ago no problems whatsoever, Origin updated and crashes. My motherboard is actually detects it as a power surge and shuts things off to protect the PC.

This happened on a prior Origin version as well. That time I took to testing heat which wasn't getting that hot (I got hotter doing other things) but cleaned my fans on GPU, Case, CPU and by the time I done that a new Origin patch was out and I did not have problems until now. I have just got done doing it again and since theres very little dust on those components I took my PSU apart and cleaned that too as it was the only area that had dust of any significant volume.

About to try Sims4 to see if that had any real effect.

Now going offline mode may work, but a better solution if you want online play might be to re-install Origin. I had to do this a month ago to get the FIFA18 demo to work, which screams to me that the Origin updater is not working as intended.

Edit: not really sure how so little dust changed much but it seems to have helped. just played 1.5 hours without a crash. You may want to check your graphics card settings (outside of game ie nvidia control panel) and turn on vsync. I did set it to "adaptive" judt dont go overboard on AA etc there as I tested these all maxed and it got my GPU working overtime, significantly slowing the game and making noise.