E18: Error in the "Digital Backbone" (CPU/GPU/RAM), this is usually caused by a cold solder joint between the GPU and the mainboard.
There are two theories to fixing this one deals with the "X" clamps that hold down the chips. The other involves re-Heating the chips. DO NOT attempt either of these if your console is still under warranty. If your console is still under warranty return it to the store where it was purchased or call MS to have it replaced.