Don't overlook the possibility that the problem could be in your router. Before I acquired an OBi, I used to use an old Linksys PAP-2T ATA for my VoIP communications, and it would behave in a similar way: It would always stay up for a day or so and then lose connectivity with my VoIP provider. Sometimes it would last a few days before failing. Without going into the nitty-gritty details here, I finally was able to prove the problem was due to a bug in the UDP NAT table portion of my router. Upgrading the router resolved the problem.
Does rebooting your router also solve the problem temporarily? If so, does your router support SIP ALG? If so, turn it off. If not, or if it makes no difference, try another router to see if that changes the behavior.
Good luck!