Marco, I would suggest putting the original 3.0 ROMs back. It sounds like you have been very careful to orient the chips correctly, so I assume you have done this correctly.
If the 3.1 ROMs you bought are indeed on flash roms, then it is possible that the chip has been erased which is why the Amiga won't boot with those chips installed.
If the Amiga works with the 3.0 ROMs installed then send the 3.1 ROMs back!
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moto
