That looks really good, well done.
If anything it looks as if the servos are struggling to hold position meaning they do not have enough torque.
If you beleive that you are sending smooth values to them, one moderate risk solution is to increase the servo refresh frequency.
This has the potential to shorten the lives of your servos, but will increase thier holding torque - theres always a trade off.
If you would like to try this I have a library which is similar to the existing servo library but which allows the refresh rate to be increased.
First find a way to check that the values you are sending are smooth, then if you want to try a higher refresh rate, let me know.
Duane B