[Asterisk-Users] SIP to SIP echo problem

[paradise dove]

I have a dual xeon server with 2 MB of ram running latest CVS *
all calls are SIP and mu-law  is the default codec for all connections
my cpu power has not ever reach above 30% of its load. everything works fine.
the problem is an echo on caller side of a call.  but this is not an
always event,
it occurs random.

what could cause echo on sip connections? 

please help
thanks,
Paradise Dove