Hi imaginableart
Sorry to hear you're having intermittent issues with service drop outs.
There are a number of reasons as to the cause of these kind of issues and I see that the modem has been replaced to try and narrow down the cause. intermittent issue are some of the hardest to diagnose and resolve due to the issue not appearing to be there all the time and in some cases only under certain conditions that are often localised.
At this time the NBN support is offer from the following contact - 1800 834 273
I do hope the issues get resolved as soon as possible
-Ashley