asterisk users - Sep 2009 - Intermittent metallic voice SIP->ISDN ISDN<-SIP

Hi all,
 I'm fighting with a really strange problem that is really busting me.
I have an asterisk 1.4.22 ( from a trixbox 2.6.2 ) and mISDN 1.1.7
3 extension on hardphone and 3 extension in softphone ( zoiper )

What happens is that sometimes the people on the other side of communication
hear my
voice as metallic and chopped. This happen either on incoming call than on
outgoing
call.

If I keep the call up ( asking the other part to wait ) for a minute or less,
then
the voice get better and we can continue the call. On the other hand, if I
hangup
the line and call again, everithing is fine.

For what I have read, it seems that the problem get triggered by some jitter,
also
if I can't understand why, being my asterisk in a local lan switched
100mbit.

I have searched through a lot of messages and info, and tried a lot of suggested
solutions, but can't get the right fix to this.

Do you have any hint ?

Thx
Pigi

The isdn is connected with an HFC-PCI card:
03:00.0 Network controller: Cologne Chip Designs GmbH ISDN network controller
[HFC-PCI] (rev 02)


this is my sip general part (jb enable to get the jitter buffer working):
jbenable = yes
jbforce = yes
jbmaxsize = 200
jbresyncthreshold = 1000
jbimpl = adaptive
jblog = yes


This is the relevant part of the misdn-init.conf
card=1,hfcpci
te_ptp=1,2
poll=128
dsp_poll=128
dsp_options=0
dtmfthreshold=100
debug=0

And this is the misdn.conf
[general]
misdn_init=/etc/misdn-init.conf
debug=0
ntdebugflags=0
ntdebugfile=/var/log/misdn-nt.log
ntkeepcalls=no
bridging=no
l1watcher_timeout=0
stop_tone_after_first_digit=yes
append_digits2exten=yes
dynamic_crypt=no
crypt_prefix=**
crypt_keys=test,muh

[default]
context=misdn
language=en
musicclass=default
senddtmf=yes
far_alerting=no
allowed_bearers=speech,3_1khz
nationalprefixinternationalprefix=0
rxgain=0
txgain=0
te_choose_channel=no
pmp_l1_check=no
reject_cause=16
need_more_infos=no
nttimeout=no
method=standard
overlapdial=yes
dialplan=0
localdialplan=0
cpndialplan=0
early_bconnect=yes
incoming_early_audio=no
nodialtone=no
presentation=-1
screen=-1
echotraining=no
jitterbuffer=4000
jitterbuffer_upper_threshold=0
hdlc=no
faxdetect=both
faxdetect_timeout=5
max_incoming=-1
max_outgoing=-1

[isdn]
ports=1,2
context=from-pstn
msns=*