My problem seems to be resolved by increasing the ram voltage, my kit has a default voltage of 1.35V and I had to increase it to 1.40V (1.43 reported by Everest) to stabilize my OC (160mhzx10 with my first 4GB) and now I have raised it to 1.45V with 8GB !!!
I think there is a lack in the voltage memory management or in the memory controler on this board.
And there is sill a pb with the reported latency values, 7-8-7 in the bios ===> 8-8-7 in Everest or HWinfo32