OK... I think I may see the problem here. Each REGISTER adds one line:

Contact: <sipaveN900@xx.yy.zz.aa:57599;transport=udp>;exp ires=0

... and the packet gets longer and longer until it's chopped off and invalid. At a 2 minute refresh, it shouldn't be rolling around different port mappings. Even if it was, it should only keep a limited number of the Contact: lines is subsequent packets.