The RAM is OCZ3G1333LV6GK - my bad. I have updated my original post.
on the label it reads: PC3 10666
9-9-9@1.65v 2GB Gold Series
I have tested the RAM several times, and I have booted the system with only one block in it at a time, trying all three blocks.
For testing I use the buildin test of win7 (I think it was introduced with Vista)
Sometimes I get faults, but they are not consistant: If I run the same test 5 times, one or two may fail, while the other three are ok.
I an uncertain as where to adjust RAM voltage in BIOS. I have tried this previously, but without improoving performance, so perhaps I did it the wrong way.