Google began another major update of it’s search results about two weeks ago and according to Googleguy (a Google representative who posts at Webmasterworld Forums) this update is incorporating “3.5 improvements to search quality” and they have not finished yet. The last improvement is slated for next week and then there will be an adjustment period where search results still are not settled. That could last an additional week or two going by past experiences.
Googleguy says to take a rest from checking your site positions until then because you may go crazy watching the major fluctuations. I would agree with that- no point in getting all worked up until you know for sure how it all ends up!