hey radioboy,
I got the exact same problem with ya! my xbox came preinstalled with modchip (i dunno what brand), and i installed Evox into my Xbox and everything went well for few months...only last week, when i tried to boot it, no screen on TV but christmas light on my Xbox, with little irritating HDD sound.
ya what do u mean by D0? maybe i will open up my xbox later to check the soldering, will update later...chiou...
UPDATE: ok i opened up my xbox and found my D0 point. it looks intact with the mobo so i just touch it gently to make sure it has firm contacts, and guess wat!!! IT CAME OFF!! OMG OMG OMG!!! since i don have any soldering, so i just reassemble my XBOX and boot it, and know wat, it's the same unbootable problems, no screen and flashing yellow,green then flashing red/green...
conclusion: most probably the D0 has loosened. Last week, i have to bang, shake, hit my Xbox to get it work. The bangings and hittings very likely to get the contact btwn D0 and the cable. Now, even bangings and hitting wont work anymore.
radioboy, try check ur D0 point and resolder it, i dont have solder kit so it's the end for me. darn D0 is so small!