Reviving this to say I finally fixed the dumb thing. *knocks on wood til knuckles bleed*
In the end, I narrowed it down to two simultaneous issues: the CPU and GPU.
Apparently, I kept buying / installing from my old supply sets of defective or semi-defective CPUs / GPUs? Today, after pulling the computer back out of storage from the past few months, I tried installing a nearly NIB SiS 315 AGP card that I used only briefly when I was 13yo. It helped me push through certain tests in 3DMark 99 MAX, but still froze on the second image quality test, and still made that same Descent 3 error. I then installed the PIII 933 again, under clocked to 700 with a 100FSB, and... that seems to be the magic combination. I've even been running that menu screen of D3 that always freezes for nearly a half hour now with no issue. Tried reinstalling the GeForce 256 again and got the same issues as before when paired with the 933.
As far as I can tell, that means I've had two defective Voodoo 5 5500s, a defective Nvidia GeForce 256 DDR, and a defective PIII 500. Something just doesn't feel right about that, like there's still some missing element here - or things are just working for now and will eventually go weird again soon. I have no clue, this computer has made everything upside down in my world 😜
So, the current specs after all that headache and hardship:
- Corsair RM850x
- Asus P3B-F
- Intel Pentium III 933 1.7v (under clocked to 700 w/ 100MHz FSB)
- 256MB Kingston KGM100x64C3 SDRAM (2x 128MB)
- SiS 315 128MB AGP 4x (running @2X)
- Sound Blaster Live! 5.1 SB0100
P3B-F 1.04, PIII 1k, 512MB PC133, GF DDR 32MB + DM3DII 12MB SLI, SB0100
P3B-F 1.03, PIII 700, 384MB PC100, V5 AGP, SB0160
CP 5170, PII 350, 256MB PC100, Rage LT 2MB, ESS 1869
PB M S610, PMMX 233, 128MB EDO66, DM3D 4MB, Aztech