[asterisk-users] Strange Zaptel Problem

[nicu]

    I am running a call center with 20-30 operators  with outbound 
projects. We have an Digium Quad port E1 interface (TE410P) on an IBM 
Server running Ubuntu-server with the lastest version of Asterisk Zaptel 
and Libpri.
    The problem is that when there are about 15 or more active calls on 
the zap interface asterisk start behaving in a weird way: it starts to 
lock for a second or two every few seconds (if i enter >reload it just 
display the prompt again and the reload executes 2 or 3 seconds later) 
but the most pressing problem is that the sip softphones that the 
operators use cannot register randomly (but very often) and the call 
initialization takes a very long time (30 -45 sec) and sometimes times 
out. The more channels are in use the worst it gets. If there are no zap 
channels in use everything works perfectly in the SIP side.
    I am suspecting that during the time asterisk is locked up it looses 
sip udp packets. but the weird thing is that established calls are not 
affected (RTP works fine).
    If you have any ideas please reply.