I had a similar problem: during a marathon session of burning 17 games on to DVD+R and testing them out on my XBox with the Matrix, the chip would suddenly stop working after I test 14 of those games (load and eject, load and eject, etc.)
On the fifteen disc, the default bios would load up and not the "256k.Evx2.5.colormod.err.bin" bios that I had on the Matrix.
So I disassemble the XBox, took out the matrix and re-flash it with the "256k.Evx2.5.colormod.err.noani.bin" (I was getting sick of the "blob" animation), and put it back on the XBox. Now everything is working find again.
I do not know why the Matrix chip would suddenly stop working. I mean, I don't think the numerous ejects and loads constitute "great" movement on the XBox.
BTW, I had the Matrix chip set on Mode 2 (1 off, 2 on) -- the "always on" mode. Plus, the light was still green when the Matrix chip fails to work. That's another problem, the green light is unreliable after the installation.
If I know how to solder I would probably buy the new 11 wires X-Ecuter pro and have a more stable set up.