Hardware Debugging - Failing Memtest86+

There are a couple of threads in the different respective care sub-forums about my problem but I thought I'd bring it here to expand upon what the issue could be.

For any background information, see this thread and this thread.

In summary, my problem is this:
The PC fails memtest86+ on test 7 only.

What has been done so far:
Parts routed back for RMA, motherboard found faulty, routed back and rebuilt. Ran memtest, failed test7. New replacement motherboard and RAM routed back to vendor for RMA, tested with memtest, no fault found.

Components involved please see "My System" (with exception to GPU which is the 8600GT Fatal1ty which passed test at vendor and not the 8800Ultra that was to be installed).

I'm really out of options in terms of what I can see to be the route cause.

PSU - Tested, passed.
CPU - Tested, passed.
RAM - Tested twice, passed.
Motherboard - Tested failed, replacement tested passed.
GPU - Tested, passed

I have tried the setup with both "optimised defaults" in the BIOS settings WITH 2.1v going to the ram.

From my understanding the memory controller is on the motherboard as the CPU is an Intel E6750 so it is highly unlikely a CPU problem. It is also unlikely to be the PSU as the first RMA included the PSU and the setup was put through its paces with vista install and Prime95. CPU was also stress tested with Prime95.

If anyone has any help, it would be GREATLY appreciated. Parts going back and forth all the time is an expensive job...especially with NFFs.

I just do not know where I am going wrong. :help:

Ok, I just scanned through your previous threads so apologies if I'm repeating anything! :)
I'm just curious as to whether you ruled out the motherboard/ram incompatibility issue? It does seem strange that it's only that particular test that the memory fails on.
Just as a double check have you tried a different memory testing program?
I can't vouch for these but a quick search gives:

Windows memory diagnostic:
http://oca.microsoft.com/en/windiag.asp

or
Memscope:
http://www.micro2000.co.uk/products/...ory_Tester.htm

or even give the original memtest a go (rather than memtest86+)?

According to the Cosair site, the RAM and Motherboard are compatible.

I tried memtest by HCIdesign, which operates from OS and that failed horrendously, at 3% of 800MB of RAM address being tested over 120 errors. I stopped the test obviously.

I can only guess the RAM, Board and CPU just don't like each other. Passes test ok at vendor with just the board and RAM. I'll ask them what CPU they used to try and figure this out...

OK, vendor has tested the CPU, RAM and Motherboard together and ran memtest overnight. It passed.

So all that is left now is PSU.

I decided not to RMA the PSU, shipping costs would bring the whole testing costs to the same amount as another PSU.

If it still fails when I rebuild, I'll go back to basics and slave stuff on cardboard with wires everwhere and see what happens.

Again, any help from anyone would be most appreciated. I am at a complete loss.