MacBook Pro Restarting/Shutting Down/Sleeping randomly. Heatsink?

A few months ago I replaced my late 2011 logic board with a mid-2012 one because my logic board died . I removed the old heatsink from the 2011 board and put it on the new board. Everything fit perfectly and I put it all back together.

I am reading from other similar issues that this could be a heatsink issue. Could someone elaborate on what the actual problem might be with the heatsink or if it could be an issue with my application of the paste? I don't think it's the temperature. It never gets that hot.

Any advice is appreciated. Thanks!