asterisk users - Sep 2006 - consitent half channel loss after 6 minutes

I have a TDM2402E card calling out to NuFone using IAX2
and after couple of minutes (6 min to be exact) the recipient
can no longer hear the caller.
The caller can continue to hear the recipient clearly.

After the 6 min I continued to listen to the call and I a could
hear the other person but they could not hear me at all. After
some time we gave up and hung up. The channel never came back.

How can I find out the issue here? When the channel was lost
I tried toggling the Hold button and the other user heard a few seconds
of my voice. But coming back off hold it was no longer being heard still.

I dont think this is nufone. Something is not right how do I go about
finding what is wrong? Any suggestions.

I am using asterisk-1.2.11 and zaptel-1.2.8.

Jerry