Have you tried to switch the modules? Maybe your DIMM slot is OK, just one of the modules failed... anyway, it still seems that you will need to RMA something... - or, it would be one more thing you could try... the CPU!
Have you worked around the CPU cooler lately? Sometimes uneven pressure can cause such memory problems... I would try to reseat the CPU and the cooler... and during the process would check the CPU socket pins...