mi1ez has a good point. You might want to remove your DIMM1 and DIMM2 strips.
If this helps, then you either need to set all the RAM modules to the specs of the slowest or replace the DIMM1 and DIMM2 with two DIMM3 spec strips.
Personally, I'd go with all the modules having the same make/model specs.
If you OC, then this becomes even more of an issue.