Maybe Sandra hasn't eally been updated to tell the difference between ATA 66 & 100 (as at this point there really isn't any diff)
ATA 100 requires a 100 controller, 100 HD and 66 cable (at least my understanding is that the cables are the same). If your achieving 66 then you have the correct cable, maybe the problem is a peice of harware that is lablesd 100, but is really just a 66 (not that there really is a difference at this point).