[Serusers] ACKs losts

Hi, Sometimes I got calls terminated in 34 seconds. Acc sql dump for normal call: | 573379090 | 2007-06-13 14:01:37 | INVITE | | 573379090 | 2007-06-13 14:01:37 | ACK | | 573379090 | 2007-06-13 14:02:04 | BYE | And calls, terminated abnormal: | 573379090 | 2007-06-20 20:37:56 | INVITE | | 573379090 | 2007-06-20 20:38:56 | INVITE | | 573379090 | 2007-06-20 20:39:59 | INVITE | | 573379090 | 2007-06-20 20:40:59 | INVITE | | 573379090 | 2007-06-20 20:42:46 | INVITE | --------- | 7631081 | 2007-06-20 20:38:31 | BYE | | 7631081 | 2007-06-20 20:39:30 | BYE | | 7631081 | 2007-06-20 20:40:33 | BYE | | 7631081 | 2007-06-20 20:41:33 | BYE | | 7631081 | 2007-06-20 20:43:20 | BYE | So, SER lost ACKs. Problem appear and disapper randomly. Config is based on nat-rtpproxy.cfg and gw-pstn.cfg Any idea?