Vista cannot start with more than 2GB ram on an Intel DQ965GF motherboard

It certainly sounds strange, since I'm running another Vista on a 8GB
workstation, but on my Intel DQ965GF motherboard neither Vista x32 nor Vista
x64 can start or be installed if I add more than 2GB of DDR-2 memory.

On that system I'm running a dual-boot with WinXP x64 and Vista x64, and
I've been able to use both for a few months with 2GB in Dual Channel mode.
But as soon as I've added another pair of sticks to get 4GB (the max ram
available with DDR-2 800 DIMMs), Vista has stopped working, by hanging
forever at the startup screen.
I've tried several combinations on the memory banks, to test each DIMM, but
Vista accepts only up to 2GB (it doesn't matter if the older 2GBs or the
last ones, or mixed).
A problem with the motherboard, one might think, but no: WinXP x64 sees all
the 4GBs, and starts successfully each time.
Even a memory test made from the Vista setup DVD has reported no problem, so
? What could be wrong? Intel says that the board has been certified for the
Premium Logo with 4GB of ram, and the memory comes from the recommended list
from Kingston.

Has anyone else experienced such kind of issue ?

Thanks,
Andrea