asterisk users - Aug 2006 - Choppy calls on IAX trunk but no problems on internal calls

We are running the default asterisk package on Ubuntu Dapper.  Our
connection to the PSTN is over an IAX trunk with our provider.  We are
getting really bad call quality on calls over the IAX trunk--voice
seems to be garbled or out of order and often completely breaks up.
But on internal calls between extensions, even with call recording
turned on, which goes through our asterisk server, everything sounds
fine.

We also have some test SIP accounts with our provider.  Phones
connected directly to our provider on these accounts have no problem
either, so we are confident that our network conditions are good and
QoS is working properly.  We are also confident that our provider is
not the problem, since the phones that connect directly to our
provider are working fine.

We thought the problem might be hardware related, so we tried three
different machines on it, each with adequate CPU, memory and disk
performance.  Every machine had the same problem.  One of the machines
we borrowed from our provider.  They were using it with a hardware PRI
and said their zttest results were consistenly 99.99 or greater and
the server had performed great for them.  But with our Ubuntu
installation and no hardware, the same server gets results around
99.92.  In fact, every one of the machines we tried got fairly bad
zttest results, although we have discovered various info that indicate
that zttest might not be a very accurate test
(http://bugs.digium.com/view.php?id=4301

We are running the default asterisk package on Ubuntu Dapper (which
has the advanced timing options used by ztdummy).  Our connection to
the PSTN is over an IAX trunk with our provider.  We are getting
really bad call quality on calls over the IAX trunk--voice seems to be
garbled or out of order and often completely breaks up. But on
internal calls between extensions, even with call recording turned on,
which goes through our asterisk server, everything sounds fine.

We also have some test SIP accounts with our provider.  Phones
connected directly to our provider on these accounts have no problem
either, so we are confident that our network conditions are good and
QoS is working properly.  We are also confident that our provider is
not the problem, since the phones that connect directly to our
provider without going through our asterisk server are working fine.

We thought the problem might be hardware related, so we tried three
different machines on it, each with adequate CPU, memory and disk
performance.  Every machine had the same problem.  One of the machines
we borrowed from our provider.  They were using it with a hardware PRI
and said their zttest results were consistenly 99.99 or greater and
the server had performed great for them.  But with our Ubuntu
installation and no hardware, the same server gets results around
99.92.  In fact, every one of the machines we tried got fairly bad
zttest results, although we have discovered various info that indicate
that zttest might not be a very accurate test
(http://bugs.digium.com/view.php?id=4301), but it is the only
benchmark we know of.

We suspect there may be a problem with with the build options in the
kernel or in the default asterisk package on dapper, so we are trying
out trixbox at the moment.  In the mean time, does anyone else have
any suggestions?  Are there some specific build options or kernel
flags we should try?  Are there any other approaches that someone
might recommend?

Thanks in advance for your time.

Carl