Something I am finding quite frustrating when trying to check serps is they seem to be region specific. If I search google.com from my computer I get different results than if I search from a proxy. I know this is not just a different data center issue as I also use the multiple datacenter google search at SEOChat.com, which typically results in all 12 searches the same as the one from my computer, so it seems to incorperate your location too. So I think it is largly based on which contry you are searching from. Is there a good tool that will show you your google serps from a range of locations?
Here's a couple: http://link.ezer.com/tools/google_serps_rank_checker.asp http://www.seologs.com/data-center-check.html
https://adwords.google.com/select/AdTargetingPreviewTool google have it as an adwords tool, but it shows you the serps from any country too.
I use a link exchange service which gives me for each text link I offer for placing it on other sites the position and a graph with SE placement history. You only have and text link for the keyword you want to track and the url on your site and the SE positioning will be tracked for Google, MSN and Yahoo. The link exchange service is free for a limited number of link exchanges but for the SE tracking option and unlimited number of link exchange you've to pay a monthly fee of $49.