Mixmonitor bad quality

Hi People!

I am using Asterisk + Freepbx in a Centos 5.5.

I have a queue with recording enabled. I tried wav and gsm formats. Both are saving in a bad quality (noise, choppy the call).

Did Anyone get this situation?

How could I improve my call recordings quality?

Is it a CPU or I/O disk problem?
Could I give more priority to Mixmonitor process?
Nice MixMonitor?

Thanks alot,