[Users] Strange REGISTER Problems

Dear List, I can register my ATA'S (Some are NATted and some ara Port Forwarded) to SER (on Public IP) and i use MySQL auth/acc together with Mediaproxy. All users can talk to others, and call them , receive calls from others no problem. But most of the time, they seem unregistered and this happens suddenly, i'm sure that Internet Connection is fine with SER Server and also clients are connected through Cable Modem or ADSL Connection. I unplug or unpower some ATA's, wait a bit like 30 minutes, the devices becomes cold or what's happening else, they can register and works perfect... But when i connect them for hours, they unregister themselves, i do not know what happens... I've tried both Zyxel P2002's with several firmwares, Zyxel P2000Ws, Cisco ATA186's and some LinkSYS Pap2-NA's but all same... Is there any chance to be something wrong in SER settings or Operating System ? Thanks, Ozan