AWS EC2 setup using sometimes internal, sometimes external IP for host candidates

Hi, I have followed Amazon EC2 <https://freeswitch.org/confluence/display/FREESWITCH/Amazon+EC2?src=contextnavpagetreemode> to setup latest Freeswitch on an EC2 instance with an internal and public IP. When I dial out to an external SIP endpoint I can see sometimes the internal IP in the candidates list, sometimes the external IP - same configuration, just retrying the same endpoint. Internal IP: a=candidate:5446536444 1 udp 659136 172.25.36.61 29668 typ host generation External IP: a=candidate:1181434720 1 udp 659136 13.238.218.223 20132 typ host generation 0 How can I debug this and does anybody have an idea why this is inconsistent with the same config when redialing same endpoint? Thanks for your help Sven