In lots of cases when fluctuations like this occur google is messing around with either the way it scores back links or is actually adding and removing links from its records.
For example tons of people reported similar fluctuations around the June 18, 2009 update from google that they are now able to index content from some SWF files. People noticed jumps that occurred on an hourly basis and it was felt for a while before and after that date.
This being said your situation could be related to an internal test or batch google is running. The massive fluctuations are too much to blame on you hitting different data centers on your search.
Long story short it is really hard to tell exactly, but most likely google is either just messing with you

or they are testing out a new wrinkle in their algo.
Update:
Just remembered I had a ton of new links show up in my webmaster account yesterday. Did not think about that until after I posted this. So maybe it is related to what I said above.