E79 is 99% of the time related to Jtag wiring. And since everything was working fine software wise before, the only thing remaining hardware wise is the Jtag wiring. It may have happened that the Jtag wiring, although working, was dodgy and has now stopped working.
How about you open the Xbox360 and post some pics of the install. Also, if possible, check for continuity between the Jtag points with multimeter.