Hi William,
The 8921 and 8920 have basically the same eeprom so they can be interchanged. We have had several cases of "non-recoverable firmware error" on boot up due to corrupt eeproms. I don't recall ever having that due to a defective controller board.
I also recommend you check the eeprom programmer to make sure it isn't "write protecting" the programmed eeprom. We have an eetools Chipmax that defaults to write protect the eeprom. The 8920 (21) writes to the eeprom every time you use it.
Rick
