Bundabah and Nerong are two locations that the summer heat will cause signal to go from perfect to zero.
Simply we have obstruction between the transmitter and almost all locations in these areas so signal can get there but it’s not expected to be stable. The signal becomes attenuated by obstruction and also free space with all locations but in these worse.
The intermittent cause is two types of atmospheric conditions. Clear sky warm days when noise level will increase due to co-channel transmitters and also heavy rain where “rain fade” will be many times worse than other locations.
No antenna upgrade can increase the levels beyond what is required to overcome these two variables and VAST is the most reliable option.