The Serpent Rider wrote:

Later drivers removed some clever "optimisations" in the Nature test.

Aha ! But I thought those optimisations worked only in 3dmark2003 ?

No, Nvidia also cheated in 3DMark2001. You can notice that tree leaves in that test have strange janky animation.

Hi again,
The problem is back -- now on ASUS TUSL2-C.
AGP4X set in BIOS, driver installed (from CD) but RivaTuner won't allow to fix it because of non-VIA chipset.

What should I try now?

Fixed by setting FSB speed to 140 MHz (was 150).

I made a mistake of installing latest version!!! beta!!! BIOS and due to it being too new couldn't install custom one (from x86-secret -- based on older non-beta BIOS) where AGP4X issue is seemingly fixed even when overclocking is enabled. Their i815 tool which should enable AGP4X from Windows didn't work unfortunately (or maybe I hadn't thoroughly read the manual)

Updated BIOS to custom version from x86-secret.
150-150-37, AGP4X.