I track my keywords so often that I can really see the change happening ... I wish it was more often with Yahoo and Bing.
dont watch the serps its like stocks will drive you nuts if you look at it all the time. Just constantly ad backlinks never ending process. hard to get ranked then you got to work even harder once you are.
Google SERPs is constantly updated and never reliable, especially for high competitive keywords. This is pretty common anyway. I would recommend you observe on your link quality and the trends of it instead of paying too much attention to the change in SERPs.