Things to consider. If power and MOBO quality are good then it's down to attenuation. This is where resistors on the ends of the wire traces and voltage drive keep the signals tight like the tension on a tight rope. Put four people on it and the rope sags.
Using lower voltage DIMMs may help.
If block copy fails, this may be something you can fix with timing.
Since you already have the DIMMs then running them with the 2nd and/or 3rd LAT timing a little looser. If you bump those numbers also bum the other 2 to 3 points. IE 9-9-9-24 would become 9-10-9-26 or 9-10-10-27. For the amount of RAM you are running and the fat L3 cache, these looser timings will be a better option than de-rating 1600s.
Oh, and don't forget to move the 5th number TRC (bank cycle) to 36 or 37.
