Hello Ben.
Unfortunately I have also noticed this on a couple of systems, not all, but exactly as you describe, sometimes it can be few months, other times a few days. It seems there is a reliability issue with the MBUS variant of the SDM120. I tried a few different things such as slowing down the readings and reducing the number of retry attempts but no luck in getting better reliability. I also haven’t been able to reset the meter when it has crashed via a software fix, as you say it requires a breaker reset. We’ve started to recommend installing the Modbus version with a separate modbus reader, that one seems to be rock solid from our experience so far. I realise I need to update our documentation to reflect this, I will do that now. I wonder if the supplier might take the MBUS version back and exchange for a modbus variant?
Sorry I should have posted up here about this sooner, I appreciate that the issue is more than a bit frustrating!