Asrock 775dual-vsta and ddr2 ram

Hi, my first post in a while.

I just built a new system from ebuyer and used the following motherboard and ram:

http://www.ebuyer.com/UK/product/114742

http://www.ebuyer.com/UK/product/82244

Upon running memtest I encountered many errors and assumed dud ram...

So I have had the ram replaced and got the same errors again. Hence I am now on my 3rd return of ram which I installed very carefully with anti static straps etc and still the same errors!

I read on the ebuyer reviews that someone else had trouble with ddr2 ram and this mobo and am asking if anyone else has had these problems?

I stuck in 2x512 normal ddr400 and it ran memtest overnight without errors.

I have upgraded from bios 1.5 to bios 2.6 and tried some of the settings but not sure which I should be playing with...

I also tried the other ddr2 slot with the same results- am I doomed to downgrade to ddr or should i replace the mobo? Others have had this same problem with branded (corsair) ram...

Any help would be appreciated! If you need any further info just ask for it- i wasn't sure what was really needed since its only a ram/mobo problem!

UPDATE:
I have tried:
manually setting the ram timings (4-4-4-12-2t)
enabling flexibility option
defining single channel
defining dram freq at 266
setting ram voltage at low/normal/high
reinstalled win xp sp2 +updates

still no change  

Thank you in advance.