8gb RAM problems on vista64

I'm having repeated system restarts since going from 4 to 8gb. The crashes
occur whenever I push the system to actually use more than 4gb, otherwise
it's rock solid and never crashes. The RAM consists of 4x2gb stalks all
purchased in one go from Crucial and is listed (by them) as compatible with
my specific motherboard. Each stalk works fine on it's own or in tandem, but
as soon as I move to all 4 the problems start. Sometimes I see the BSOD with
"stop" messages, but most often the system just reboots immeadiatly.

Some specs;
CPU: Intel quad core q6600
Motherboard: P5N32E-SLI
RAM: Crucial. Part #: CT2KIT25664AA1067 • DDR2 PC2-8500 • CL=7 • Unbuffered
• NON-ECC • DDR2-1066 • 1.8V • 256Meg x 64 •
GPU: Nvidia 8800gts (640mb version)
OS: Vista Home Premium 64-bit SP1

I've trawled the hotfixes here, and on the manufacturers website for each
component but haven't had any luck. My drivers, BIOS and Vista updates are
all up to date, i've even tried a clean format and install of vista. Can
anyone out there help?

Fingers crossed,

Cameron.