[Serusers] nathelper silent at start of conversation

Hi all, I'm running ser and rtpproxy on a public IP, I have several clients registered that are behind different NAT firewalls. I have been having the issue that occasionally when a user makes a call they the called (or sometimes the callee I believe) gets a period of a couple seconds without audio, the line is dead, and then after 5/10 seconds. Has anyone come across similar problems? I have had this issue with just the basic nathelper.cfg that comes with ser, and running ser from the 0.8.14 and CVS HEAD branches. rtpproxy is the latest from cvs also, running with no extra flags. It appears that rtp gets sent from UA1 to rtpproxy and gets dropped by the NAT in front of UA2, this continues for a couple seconds, and then a RTP packet comes from UA2, and two way rtp is established. As I said earlier this is an intermittent issue. I have a full network trace of this behaviour which I will be happy to provide on request. Thanks, -Jev