Jittery sound

I am experiencing Jitter in recorded sounds.
I am using SIP only configuration with no Zaptel card.
I have enabled ztdummy module.

Now the strangest thing is that I have noticed that when I was updating my changes in FreePBX, the voice suddenly got fine. It seems this happens any time if there is some activity on the server. When I ssh in and run say ls -R on /, while the command is running I am getting the correct sound. After the command ends, I start to get the same problem again.

could any one give me a pointer.