Different brands?? I used to mix n' max my ram and it caused me to crash a few times. It's best to use the same brand/model of ram if you can help it.
another thing is that there could be some weird anomaly where the rated speed difference could be messing with the stability of one of the sticks.
I don't experiment with RAM a lot so i am just theorizing.
mixing different memory capacities is likely what is causing your issues. Try just using the 2x4 kit until you can get another 8gb stick. whatever the lowest capacity is what the system will assume the other sticks are. your 8gb stick could bee read as 4gb and then thee data loaded from the disk could be misplaced due to the assumption of capacity.
again, i'm no expert, but that is my idea of your problem.