That type of output can occur on systems which are too fast, but it can vary from card to card. I have one Voodoo which does the same with speeds faster than 633MHz/66FSB.

Try a slower system and keep the FSB under 100MHz. A Pentium II @ 333MHz/66FSB is ideal, more or less.

Huh, that's exactly the kind of obvious-in-retrospect feedback I came here for, thanks Meatball 😁
I have a Pentium MMX 166 in preparation to be bought and taken home from work, so that fits nicely and I should be able to test the theory soon enough.

Two questions:
1) Any chance it would help to cripple/slow my current CPU (Athlon 1100) down by disabling cache(s), or does that not do what I need because it leaves the FSB untouched?
2) You think the CPU speed problem could also be the culprit in mojo.exe detecting too much FBI memory, or do I more likely just have two separate issues?

1 - Using slowdown utilities like throttle or Setmui combined With a slower FSB is worth a try. Give it a try no matter what; some cards are luckier than others. For a proper mojo test, I would not rely on the results unless the card was tested at speeds known to work with the Voodoo like your incoming Pentium MMX, for example. Install the card there and see if you can reproduce the problem.

2 - A suspect pin is a concern. You can use an exacto knife to gently apply pressure to determine if it’s loose. Be very gentle, though. But again, I’d validate the card in the MMX 166 machine, first. Unless it is something obvious like pins touching, I’d avoid taking unnecessary risks in damaging the card, however slight.

If You can reduce the multiplier to 5 or 6 it may work. My Voodoo 1 failed with the rectangles on screen after 600mhz with an AMD Duron or Athlon, but failed at 500mhz with an Athlon XP Barton,
I think the extra cache had an effect as well.