I would advice to use simplot http://www.negtronics.com/simplot (or something similar) to look at the values you are sending to the servo's.
At least then you know whether the problem is in your code or in the electronics/mechanics.
If you have a scope you can also read the signals send to the servo's. I would also check the voltage. I would not be surprised you have voltage spikes because you have 12 servo's.
Best regards
Jantje