Unable to create channel of type 'ZAP' (cause 34 )

Helo community.
I am new to asterisk, and my first installation got stacked.

Several times per day I am getting message for all calls:
WARNING[6273] app_dial.c: Unable to create channel of type ‘ZAP’ (cause 34 - Circuit/channel congestion)

And all incoming calls are dropped, current calls still work.
I need to restart zaptel, or run “zap reload” several times to avoid call drops.

System info:
OS: CentOS release 5.2 on 2.6.18-53.1.19.el5.028stab053.14
[color=red] load average: 1.80, 1.81, 1.79[/color]
Mem: 3889832k total, 1417220k used, 2472612k free
asterisk-1.4.21 (also tried older version - 1.4.15, same error)
libpri-1.4.5
zaptel-1.4.11
Device: wct4xxp+ d161:0420 Wildcard TE420 (4th Gen)

/etc/zaptel.conf:
span=1,1,1,ccs,hdb3,crc4
bchan=1-15,17-31
dchan=16

/etc/asterisk/zapata.conf
[channels]
language=de
context=default
signalling=pri_cpe
switchtype=euroisdn
;If you connect to a hicom PBX set your ISDN Numbering Plan Identifier to unknown.
usecallerid=yes
hidecallerid=no
callwaiting=yes
usecallingpres=yes
callwaitingcallerid=yes
threewaycalling=no
transfer=no
cancallforward=no
callreturn=no
echocancel=no
resetinterval=3600
echocancelwhenbridged=no
rxgain=0.0
txgain=0.0
group=1
callgroup=1
pickupgroup=1
immediate=no

group = 1
channel => 1-15
channel => 17-31

/etc/asterisk/extensions.conf
[default]
exten => _X.,1,Dial(ZAP/r1/${EXTEN},60)
exten => _X.,2,Hangup

today I discovered that eth0 and wct4xxp are on same IRQ.

cat /proc/interrupts |grep wct
169: 17122 357944793 IO-APIC-level eth0, wct4xxp

but fixing IRQ
169: 17475 25439223 IO-APIC-level wct4xxp
by disabling eth0 (I used eth1 which is on IRQ 58 ) DID NOT fix the problem with call drops.

Please help.