Well, after several days of stability following the upgrade, I started to experience the huge drops again two nights ago and my IP profile suffered. I'm hoping that was a one-off though, as it's improved again, but not as stable as it was immediately after the update. Go figure.
It does seem that this chipset is particularly sensitive to interference.
Here is a graph from earlier today. There are four extreme drops in the noise margin, and I have managed to isolate the cause... the kettle! I noticed the correlation, tested it and, sure enough, every time it boils, my SNR drops
Would something like a surge protecter help with this?
Anyway, I still feel there is room for improvement, so have just ordered a Thomson 585 v6 from ebay. I'm curious to see the difference.