I have 2 towerpro mg995 servo connected to arduino witth 12v 5A supply.the pwm pulse is coming from arduino using PWMServo library. The circuit is attached . When both of the servo are connected sometimes there is minor jitter in servo. What could be the reason?

what is the code you are using? the values you defined for servo needs to be cross checked, then it works fine

Most jitter happen due to fluctuations in power supply,  add filter capacitors

Look for your LM317 current rating. It cannot deliver sufficient current during loading and servo jitter occurs. As a Adjustable regulator, please increase the supply voltage to 6 volt and then check the performance.
If still jitter occurs, just check with another servo.
Either servo or power supply is faulty. If servo found, you can try adjusting the internal preset if any.
You may try using an buffer stage in signal line too to figure out the culprit...

Waiting for your reply.

That is Right. Add Filter Capacitors .. mine Worked Fine after adding capacitors