My best guess: Via's AGP bus implementation is just wrong somehow. So far, I've used two boards based on the very-much related KX133 chipsetthough neither with T-birdsand I've seen lock-ups in 3D apps with both of them. The problem seems to be related to the CPU clock frequency, but it's independent of overclocking. I've "tested" this problem more extensively that I'd care to admit trying to make things work right on a couple of systems here, and I've used both mildly overclocked CPUs and an 800MHz Athlon at its stock speed (and below). AGP-related lock-ups occur at CPU frequencies above about 700MHz. Turning off AGP 4X, fast writes, etc. helps, but it's no cure.
Then there's the still slightly mysterious problem with the KX133 chipset that caused AMD to refrain from making Slot A T-birds widely available via retail. Speculation about the nature of the KX133/T-bird problem has centered on the way the CPU signaling is handled, but I tend to think there's more to it. Maybe the KT133 is betterit may handle CPU signaling rightbut I don't think it's only problem. The "fixed" KT133 seems to be having AGP lock-up problems, as well.
The long and the short of it: perhaps neither of Via's Athlon chipsets is right.
Anyhow, that's my take. Maybe it's just frustration talking, but I've tested enough different ways and isolated enough different variablesyanking out sound cards, swapping video cards, installing newer/older driversto be convinced. For now, I'd recommend an AMD 750-based mobo or (egads!) an Intel-based system instead of any Athlon/Via combo.