You have already gone part-way along the line that is usually necessary to troubleshoot the 68K Macs: sort out Extensions, and beat them into insensibility, or submission, with minimal peripherals and and add-ins while you do so. You'll notice that the linked page recommends that technique in this context, too, and points at the video system if conflicts continue when Extensions have been sorted. Is the video in a IIfx in a pseudo-NuBus slot (built-in) or a real one?
64-pin RAM being what it is (ie, uncommon, and mostly in 1 and 4MB cards), have you enough that the MLB can continue to doddle along when the Rocket has finished booting? There has to be enough for AppleTalk-over-NuBus to be maintained (for communication between Rocket and MLB) as well as basic life-mechanisms on the MLB, which would include video if the IIfx uses Vampire Video. (I don't have a IIfx, so this latter remark is just rambling aloud.)
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64-pin RAM being what it is (ie, uncommon, and mostly in 1 and 4MB cards), have you enough that the MLB can continue to doddle along when the Rocket has finished booting? There has to be enough for AppleTalk-over-NuBus to be maintained (for communication between Rocket and MLB) as well as basic life-mechanisms on the MLB, which would include video if the IIfx uses Vampire Video. (I don't have a IIfx, so this latter remark is just rambling aloud.)
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