asterisk users - Jun 2005 - Intermittent audio issues with Asterisk behind symmetrical firewa ll

I apologize in advance for posting this yet again (3rd time actually). But I
have a little more data to share this time so bear with me.

I have Asterisk running on an internal IP address behind a Cisco Pix 515
with firmware version 5.2(3) 

Here is the setup
 
Mitel SX200 PBX --- Asterisk --- Cisco PIX --- Internet

I have statically mapped the external IP address to the internal IP address;
and created a rule for udp packets inbound to the external IP address from
any host bound for port 5060 be permitted.

Asterisk has the default 10000 - 20000 setting in rtp.conf as well as the
external IP address set in sip.conf

Roughly 10% of the time there is no audio making it past the firewall in
either direction. I have tested using ztmonitor to monitor the channel and
can see audio on the internal side (rx) but nothing on the external side
(tx). From this I make the assumption that the audio is being passed from
the legacy PBX to the Asterisk but being dropped at the firewall.

It is my understanding that I do not need to explicitly allow the rtp
traffic over a set port range since the PIX will automatically open the
required ports due to the negotiation in the initial invite/ok. 

I am all out of ideas for what to do in this scenario. I have searched high
and low and found nothing that has worked.

We are in a pinch to make this a production server to handle inbound phone
calls but have to fix this 10% failure issue.

Thanks in advance, hopefully someone has seen this setup before and found a
solution.

Cheers,
Geoff