Is there any chance that this is unrelated to the SoC or the RAM themselves? I.e. could there be a capacitor or voltage regulator in the power circuit somewhere that works until certain conditions are met (temp? current high/low?). Maybe you got a bad batch of -a- component that isn't working within spec X% of the time? I know that there was a lot of talk about whether or not to use Tantalum capacitors - could be completely unrelated of course. A lot of the features on these boards are -really- tiny. Finding the slightly bent needle in the needle stack could be frustrating - if there even is one.