I had exactly the same problem.
My EDRAM temperature was shown as very high. Right after the start of the console.
And others were on normal levels.
The problem appeared because Microsoft used Pb solder for Xbox 360. After many heat->cool down cycles it collapses. And creates higher resistance on thermistors which measure temperature ( they work in a way - higher temperature - higher resistance). So the temperature was false high.
The solution is to solder resistors contacts ( to renew the structure of solder) r4d3-r4d6. (side by side).
This will remove additional resistance and solve the problem.
