Widescreen 1680 -1050 Resoloution Problem

I have just installed Windows Vista Ultimate 64bit edition on my newly built pc.

Spec:

Mobo: Asus P5E VM HDMI
CPU : Q6600 Core 2 Quad
Ram : 4gb (4x1gb) OCZ pc6400 (800mhz)
Video Card : Asus GeForce 8400 GS
Screen : Acer X223W 22'' Widescreen TFT
My problem is that even tho I have installed the mainboard drivers, video card drivers from nvidia latest 64bit drivers version 7.15.11.7519 (date 16/05/2008)
It does not support 1680-1050 resoloution for my widescreen.

BUT when I was running Vista Home premium it did !!

That resoloution is not listed when you goto windows display settings.

I have been to the Nvidia control panel and Manually Added this resoloution and it accepts it..

but when I rebooted it went back to 1600 - 1200

Any help appreciated.. Thanks :confused