80% of the bloggers who gets >75% of traffic from the search engines, predominantly Google; obviously has noticed the curve goes up and down, up and down like a sine wave curve.
It does not feels very funny when the curve is going down, because you do not know where it will stop.
You have discovered a keyword combination (or better to be said a keyphrase) that was making you get few hundred visitors per day. Suddenly, another website has pushed you down. It happens. The structure of Internet is very dynamic itself.
This one aspect of the reason behind going down. Another big problem is random posts like plugins. They changes the links with time, somewhat cyclically. But you can not live with it too. You have to shuffle the old posts.
Stabilize the number of visitor
For WordPress users, there is plugins to show the incoming search terms for that article. We are using SEO SearchTerms 2 ; which is free and it really stabilizes the rate of fall. What it does, it stores the top keywords for a certain period for that post and keep the link below the post. You can use for sidebar too. It is a fantastic plugin. Our headache of examining visitors hourly trend has gone now. Another thing it can do is, it automatically reposts the old posts. It was good if it had some settings to control which Categories to be excluded.
For non WordPress users; we will suggest to retweet manually the old posts.
Another trick is; post relatively less frequently when the curve is going up; post more frequently when it is going down. It will cause a rapid rise and back to a bit higher level and keep that level.