what would be a good way to debug this?

[vale sapphire]

Why does it suddenly start writing to disk? Seems like that change corresponds to the drop in bandwidth, but your description of the issue doesn't mention it

Right, so it drops connections (for unknown reason) then starts dumping huge amounts of text.

If it's connecting to a specific type of server - ie Elasticsearch - you might want to start debugging by looking at the logs on the other side.

If not, and it is falling over after X hours, I'd be interested if it's an issue with your connection if it's locally deployed.