Thanks @Keith2, yes that clears things up a lot. I think the fact that the inverter is well covered, on the east wall, reduces the likelihood of overheating, as well as what you are saying about the time of day it is occurring.
What you are saying is concerning because I would say that the vast majority of new residential solar installs in Australia, oversize the inverter with panels. There are good reasons to do so, we talk about why in our Guide to Buying Solar here: Oversizing your inverter.
For that reason, I am interested in getting to the bottom of your issues Keith, because I know that generally, oversizing the inverter does not cause problems like what you are describing.
I am not a solar installer or electrician, so hopefully someone with more inverter technical knowledge will be able to wade in here. I’ve had a look at the Solarmax 4000p data sheet, and it looks like the shutdown voltage is 600V. It’s possible that if all 18 panels were run together in a single string, then the system is reaching this limit and shutting down. I’m just guessing here. I guess it’s also possible that there is some limit built into the inverter that shuts it down when it reaches 4kW, but that would be very unusual from my understanding.
Keith, I would recommend getting an electrician with solar experience to take a look. You’ll need an electrician at some stage either way to fix this, I think it’s worth getting them involved before buying a new inverter. Rather than getting a new inverter, they may be able to do some rewiring. Which won’t necessarily be easy, but installing a new inverter might mean getting the system inspected again, and that could complicate the whole matter even further.
Good luck!