Thank you so much for your help kmanley57 and piercet! I finally figured it out!
I replaced the RAMBo and after examining the old one, I found that the right pin for the heater wire was melted off of the RAMBo. Pretty scary/surprising stuff. It’s possible to fix, but just wanted to add this info for those of you looking for solutions at home.
Thank you for all of your help. Pictures below for the curious.
Note the long pin on the right of the long black connector - it is marginally melted off the board. 
From the back you can see the solder has been melted on the leftmost solder pad in the line of through-hole connectors for the black connector. Crazy that those pins get that hot. Perhard the connection was just not quite good enough and heating up with all of that current passing through?
