Hi Tyler,
Thanks for the reply. Over the last couple of months we've done next to no link building - our rankings improved since penguin so kept focussing on quality links rather than quantity.
We have however been focussing on removing any links which we felt were spammy.
But it is definitely weird. It is definitely tied to the number of results Google is calling from though. Why would the total number of results available change so dramatically for the same keyword??