Problems with multicore time calculation

Hello everyone,

I am working with the attached file and there is a very unusual behavior with the second study. It is a frequency response study with a parametric sweep: I need to know the frequency response of the beam with 5000 different values of the parameters. The program is very fast for the first 10 per cent of the parameter values then it slows down and  becomes very slow as long as I must stop it (around 10 %) and restart it where the final points of the previous calculation become the first of the new one.
It does not depends on the different parametric values because of the difficult for different values is always the same, indeed when I restart the program it is again fast.
I have given a look to the CPU usage and what I observed is that as the program proceeds the  usage  percentage of the various cores  decreases, furthermore it does not use the virtual CPU.
The memory store indicated down right is 4|8 (almost).
I have this system:

MAC OS X 10.8.2
Processor: 2 x 2.4 GHz 6-Core Intel Xeon
Memory: 12 GB 1333 MHz DDR

Thank you for your help.



Kind regards


Fabio