Seems you're not the only one with that issue.
Had an NBN tech out after a service call and mine was doing the same. Soon as it starts authentication it reboots. They'd said they had a few of the same with that model recently.
Not sure about what your solution may be but in the end I went and got myself a new modem (although means no phone any more as someone in their infinite wisdom decided that TPG wold run rpoprietry connection methods and configurations not supported by most VOIP units).
