Have any of the other top 10 moved significantly?
Have you made any changes to your page in the last few weeks?
Have you lost any major backlinks?
It might be a small change in the google algo, or might just be a glitch.... if you haven't made any changes then analyse your site vs the current top 10 (drop me a pm if you need any assistance) and see how you can improve from no.8
Also check out McDar's tool here -
http://www.mcdar.net/q-check/datatool.asp - that will tell you your position across a number of Googles datacentres...