Gamers Nexus tests don't show overheating to be an issue, it could just be a defect but only time will tell.

The airflow in the case is pretty great for something with a solid front - thermally there isn't much to criticize about the steam machine. that it ran for 20 minutes and then died with an GPU post error would make me think less about a direct chip failure and more about a resistor or capacitor having a contact issue, prompted by vibrations from the fan and the previous transport, maybe even outright transport damage escalating after a short runtime.

There's a lot of power limitations that Valve put on the Steam Machine to prevent it from getting warm or needing to speed up the fans (making it louder). It's part of why the Steam Machine doesn't perform well compared to similarly priced computers

Heat is definitely not the issue