Well, I ran memtest overnight and this morning there was an error on test 5.
This is at stock memory settings 1000MHz unlinked, 5,5,5,15

The system specs are as follows:
QX6700 10x multi 1400 FSB voltage under load 1.45V
Water cooled (triple rad)
nvidia 680i board
4x2G F2-8000CL5D-4GBPQ 2.1V 5,5,5,15
Silverstone 1000W PS
All chipsets set at auto voltage

So I am now in the process of running the memory at 900MHz with memtest.

In Vista64, it seems that I can use the computer for everything including processing really large photos in photoshop (4G). If I run prime95 on 4 cores for a while the system locks hard. Can't move the mouse nothing. The same happens with linpack but much quicker.
This is something I never saw previously with my system. Normally, there would be an error in prime95 for example and that's it.

All right. It is possible that one or more dimms are defective given the failure on memtest. I suppose that if I can do 2 dimms at a time and see if there is a pattern perhaps there is a dimm that is not so good.

I do not really mind running at 800MHz, since the presence of 8G for my photoshop work is what really counts. I did some processing last night with a 48x72" 300dpi file with several layers and filters and it took about 10 seconds vs 45 minutes or so in the past with 4G.

A question: If this memory is failing at 1000MHz, is it a dimm fault or just an issue with speed with 4 dimms installed?
Thanks for your help