I would run a memory test program to verify the problem. (Or if you can't find a suitable program, a simple test is to PKZIP some files and then unzip them... see if you get a CRC error)
Go into the BIOS settings and try a slower cache timing. I remember coming across chips that didn't live up to their specified timings, but would work at a slower setting. Having two different brands of chips in there could also figure in.
