just offering more SOP for troubleshooting:

for shits and giggles, can you make sure the drive works properly on another channel or another mobo altogether. I suppose its possible for the board on the HD to be screwed somehow such that it goes to that level. Not likely.

Alternatively, or perhaps in addition to, using another HD on the same cable on that mobo to see if that works.

Maybe a pin on the HD or the mobo is bent or broken. I wouldn't suspect this behavior to cause that if that were the case, but i suppose its worth checking.

Is there a way to manually set the mode in the BIOS? What about booting a linux live cd and seeing if you can manually set the mode that way.

Are you SURE it's an 80 conductor cable?? :P

If not the drive, not the mobo, not the CMOS, and not the cable, then you've got me.