For me it has to be ranking NO1 for a debt related keyword approximately 6 months ago. I made $239 dollars in one day from advertising and had a massive traffic boost, but the very next day bounced back to number 8. Haven't yet managed to achieve no1 status again
maybe there are many competition out there to compete the same keyword you used, so the traffic flow to your site will slightly changes..I think my experience in google not so bad and i like it
I have observed fluctuation in Google rankings because Google preferring fresh content and websites load with unique content & also links.