You are using advanced GC options which might not be helping. As stated in step #5 of the tuning guide, we suggest not using these options unless there is already a problem as they might cause problems. Do you have the "-server" flag turned on following step #4?
Have you followed tuning step #6 for a 12 core machine, which is what you have?
It is also possible that you are just over-loading the machine, in which case you might need to setup Liverepeater (origin/edge) cluster to handle the load.