if it is stable at those (stock) speeds when you do not overclock, then you overclock your cpu and the ram speed to a lower speed (aka amd "divider") to run stock speed, right? and its unstable? if this is the case, either your cpu overclock is unstable, or you do not have the divider set up correctly.
set your ram to the lowest speed supported by your board, then overclock your cpu. probably your ram is overclocking too much.