asterisk users - Jun 2005 - TDM400P Channels stop answering after some time .

Hi

Need help on bridging SIP with TDM400P(4 FXO Modules )

My setup is as follows


US OFFICE ---------TDM400P(FXO) --SIP--- TDM400P(FXOs)--------INDIA OFFICE
 (DSL Line)              Asterisk                            
Asterisk                           PBX(Siemens) /DSL Line
                                  Server                              
Server                    

Everithing works fine for one or two calls or maximum 4 calls over
the setup.

Ie after some time zap channels are not ringing.Then I have to reload
asterisk.Once restart everithing works fine for 2 or 3 calls over the setup
then the same issue .I need to restart asterisk again .

Is it the problem with TDM400P ?
OR the problem with 2.6 Kernel ?
or  Problem with SIP and TDM Card ?
How I can troubleshoot ?

I am using Fedora core3 Kernel 2.6.9-1.667

My zaptel.conf on both systes:
loadzone = us
defaultzone=us
fxsks=1-4

My zapdata.conf on both systems :

signalling=fxs_ks
rxwink=300
usecallingpres=yes
transfer=yes
echocancel=yes
echocancelwhenbridged=yes
echotraining=yes
relaxdtmf=yes
rxgain=4.9
txgain=6.9
busydetect=yes
callprogress=yes
progzone=us
musiconhold=default
jitterbuffers=4

My sip.conf on both systems
[pbx]
type=friend
username=pbx
secret=pbx
host=192.168.X.Y
dtmfmode=info
insecure=very
qualify=no
disallow=all
allow=ulaw

Do you want any more details ?

thanks
-Sandeep