It is clear to me now, the artifacts you get with the IIfx and IIsi ROM must not all be the same, I don't know what determines it though. There are many different revisions of the ROMs and if you look at different logic boards, they do not use the same chips or brands. Due to these many variations I suspect that there are a number of different possible artifacts you can experience when using a non stock ROM, all of which I believe to be harmless. At least with the ROMs and logic boards I have tried, the most common artifact I have seen are the "strange artifacts". Now that I think of it, the first socketed board I had displayed the garbled checkerboard pattern when I used the IIfx ROM.

JDW, I do believe the board you have that displays the "strange artifacts" all the time most likely has a harmless quirk in it.