I don't know how iflash workes, didn't have any board that uses this flash program. Maby you can try a other revision of the program.
This problem occured months ago when I tried to make a mod by myself. It seems it is a general problem of the Intel D8XX series. I did some experiments together with Sloozer and tried many different mods. He was not able to flash one of the mods onto the chip as well. The problem occurs even if you only extract the 1b module out off the AMI core and reinsert them again WITHOUT ANY modification of the code. Uniflash didn't work as well... For SLP1.0 only try this. Install Intel ITK and modifiy a DMI entry: Use one that uses a wide valid range such as Acer, HP, Compaq or Fujitzu. Exchange the OEM files of XP, if necessary. After that save the modified bios and try to flash it. A SLP2.0 modification is not possible with ITK.