Nforce4 K8 Triton + 4x PC400 DDR DIMMS = PC333?

I've got a nForce4 K8 Triton board -- GA-K8NF-9 board with an Athlon
64 3000+.

With two 512 MB PC400 DIMMs, memtest reports the memory speed as PC400
and it is stable.

With four 512 MB PC400 DIMMs, all identical, memtest tells me that the
BIOS has the memory speed at 333 MHz by default, and it is stable --
no problems.

If I force the memory speed to PC400 in the BIOS, memtest reports zero
errors even when run for 24 hours straight, but I do see errors in
actual use -- for example, Linux will have occasional general
protection faults when under load or gcc will crash in random places.

Obviously there's something going on with the memory speeds. Any idea
what? Some chipset limitation?

--
Doug McLaren, There's no place like /home