With that configuration, my original suggestion stands. Your 500W PSU is supplying Current to several high-Current using devices. That can cause a small voltage drop in other areas, such as memory.
You should try going into the BIOS, raise the Vdimm (voltage), by .1V to 1.9V. This ensures the memory is getting it's fair share of the Current.
Test with memtest86 to verify stability.
If still having an issue, verify CPC is off (2T timing). When using 2 or more memory modules, 1T timing (CPC on) can cause instability.