G729 garbled sound please help

I just bought several license for g729 codec but i have a problem, when i enable g729 codec it start ok but after 5 minutes the sound started to be garbled then after another 5 minutes it becomes good again, however this is not happening with G711 at all. is there anything i can do to fix this? I’m using Asterisk 1.2.3 and Dell Dual Xeon 3.2 with 2GB memory, No FXO or FXS cards. i downloaded the P4 g729 binary codec_g729a.so I didn’t find the one that for Xeon processor but i’m not sure if this is the problem. Please help