I know you've already tried flashing from sd, but here's a link to an app created by Wideawake that will flash from your sd. As noted earlier, make sure your sd card is formatted FAT32, d/l the .nbh to the root of the sd card, no folder, nothing, put it in the root, run this app and let it do it's thing, see if that helps at all. Good luck, if this works, donate the $20 to ppcgeeks.
http://forum.ppcgeeks.com/showthread.php?t=111602