What is the Mushkin model number of the memory modules? Are both dual-channel kits the same model number?
When you said "... tried changing ram clock in BIOS ..." are you talking about the Memory Frequency (MHz) setting? What did you set it to?
What is the DDR2 Voltage Control setting's value set to? If it's at the default value of Normal change it to the operating voltage that Mushkin has specified for those memory modules.
I don't see any memory timings settings in the BIOS Setup section of the motherboard manual for the GA-N680SLI-DQ6.