asterisk users - May 2011 - Asterisk 1.6 - voice quality becomes poor after several minutes.

Hi all,

I have strange problem - maybe someone could help.

Asterisk 1.6 running on Debian installed on HP DL380 with SmartArray,
2GB RAM, Xeon 3GHz on board.
Just after the call is answered all is just fine but several minutes
after that voice quality becomes poor.
Something like quantization effect take place.
I have tried different codecs - no success. Network problems like
jitter or so are rather not possible.

If anybody have some idea - I will be pleased.

Regards,
Jarek