I just looked up the IIfx VRAM, only to discover that there is none soldered-in and no slots, ie it has to use a video card. So here's a quick test for you. Snitch the card from the Q 950 and try it in the IIfx. A card that works in a 950 has to be Quadra-compatible, which accords exactly with St Radius' dictum for compatibility with a Rocket.

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PS. A subsequent thought. Perhaps you are using the 950's built-in video? If so, to use tomlee59's expression, feel free to e-shred that suggestion.