If it doesn't boot into Diagnostics, You're fairly done.
That is in ROM that should not be effected by a firmware load. Sounds more like the processor isn't happy.
Not sure if anyone asked, I wonder if you have one of the real old boards that had the hardware problem with a chip coming out of it's socket. Do a search here, it was a well documented problem, and the serial number range of the effected units are listed, so you can check against your serial number easily.
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Ahh ... some dick DID change my tag line again.