I think what PS3News meant when he said it could be because of a sensor, he meant that the system was too hot for example.
The fact that you have 2 systems that have the same problem leads me to believe that it is the conditions that the console is being played in. Are you using it in a well ventilated spot that doesnt get alot of dust buildup. Or are you using an import console that has a different standard voltage input?
There are many things that could have gone wrong, but the best idea instead of pulling the consoles to bits is to get onto sony customer support and get them to sort it out. They would no doubt have better knowledge that me.