Memory bandwidth will drop dramatically like that when
1. The CPU is using less than all cores/threads - check in Aida64 that all 6 cores are in use
2. The number of DIMMs is reduced. - check in Aida 64 that all 8 sticks are listed. Further, check in BIOS GPU/DIMM Post to see that all the sticks are recognized by BIOS when set to 2400.
The most likely case is that the 3960X IMC cannot do 2400MHz. Mine couldn't and I had to back off to 2133. Perhaps one of the IMC channels can do 2400, but the others can only do 2133. Possible solutions are:
1. run at 2133 and take the lower bandwidth
2. switch to a 3970X or one of the IvyBridge CPUs that regularly do 2400 RAM - 3970X, 4930K and 4960X have worked at 2400 for me. Even with RAM easily capable of 2666, I have to run at 2400 with those CPUs and 2133 with a 3960X.
Jeff