> The symptoms you've shown are typical for bad memory;
> have you run some extensive memory checks on the hardware?
> The BIOS POST memory-counting is often not enough, there are
> free memory checkers available on the net, some of which also
> tell you which bank contains a faulty SIMM.

You can sometimes also verify this by swapping [SD]IMMS and seeing if the
failure address changes.  Not the most reliable way but hey, it's cheap!