This is a bit late since you already sent it off, but here was one of my scenerios. Hardest XBox to modify so far, I only do homebrews...
I installed a modchip and got the blinking red/green. So I removed the chip, broke the +5v to CE link on the MB, still got the red/green. Checked all connections, good.
Then I noticed if I lifted the board slightly it would work. Hmm, so I went ahead and removed all the wires from the board, same results.
So I checked the ground on the chassis of the XBox and it was .170 range, not a very good ground, so I concluded it was a ground.
My guess is that if I powered it on/off originally a bit more I would have found it wouldn't work 100% even before I put in the modchip. But can't really test that out now.
For some reason if I used certain BIOS on the modchip it would work, must be some timing issue where it waits a bit longer for the chip to enable itself. You must get ground to CE to enable to chip...
Well right now I got it working using Evox 2.5 Red Logo Error BIOS. Some BIOS work, others don't. I think if I wired up a TRUE ground it would work with all the BIOS, the XBox has a floating ground.
Hope this helps someone when they find removing there mod chip still results in failure.