Echo, Delay and Static

Hi every body, I recently update my Asterisk Box from 1.2.13 to 1.4.4, now I’m having a problem, when we make calls between extensions there is a delay in the voice and also echo, we are in the same LAN, 100 Mb, I’m using eyeBeam 1.5 and also I have SJphone for testing too, also when we receive calls we get echo in our side but the caller doesn’t but the caller says that he hear us very far away, some times there is echo too, we have a 5Mbps link and should be more than enough to handle 50 concurrent calls, but at this time we are just handling like 5 or 10 concurrent calls, the asterisk box is an HP Server with 2 3.0 GHz XEON MP processors, 2GB RAM, we have 3 VoIP providers, and the problem is with all 3, we use g729 as audio codec, I’ve been stuck trying to solve this, I did networking test, stress test in the server, rtp test, everything seems to be fine, I can try justify the problems with the providers but locally between extensions?, if somebody have had some similar problem and addressed it some way maybe can point me in the right direction.