Depending on exact location in Dunedin, this is most likely to be interference related. Which way is the antenna pointing? :)
We have a bunch of Trango gear up as well and don't notice any characteristic like the one posted.
Surveying from one particular location indicated -30dBm spikes on 3 channels (both polarizations) and one of our units managed to associate with an old AP (hey, free Internet!), so there is a lot of potential for interference.
There is an AP condition with old firmware where the heater doesn't kick in properly and causes packet loss on the Ethernet interface (that's upstream from you). A mismatch in speed/duplex settings on your end can cause this too, but that's unlikely to be time-dependent.
Some of the newer Trango SUs have firmware issues which can cause similar to what you are seeing. If you can monitor RSSI on the SU you will get a clearer picture of what is going on. It's possible that it is overloading on the AP or congestion upstream :-) We cleared up a similar packet loss issue by adding a 88cm dish for a 3km link.
