You know when it comes to the big G in the sky, I guess anything goes. The algorithms that the google bots employ seem to mystify everyone one of us. I'm no expert but I've heard that websites will fluctuate, as yours is doing, back and forth until finally stabilized.

The reasons why this happens? There must be a ton of them, but give it some time and I'm sure that your website with that many visits is bound to stay on page 1 for a long time to come.

do you have a lot of vidoes or new content...
Videos will hold you on on the first pages longer then static content.

Adding new post or pages will pull you up, then you will fall off as the content gets
older. As for the Alexa, no idea. That is a very static number based on traffic.
For it to be falling down you traffic should also be falling down.

An increase in links can pull you up, then you will slowly fall back down.

Sites with continually changing content will move around a lot.

If it's a fairly new site it can do a lot of jumping around with Google, especially if you suddenly are getting backlinks to your site.

"Google Dance", maybe.

Just keep publishing good content and consistently work on getting backlinks and your site should do well in the serps.

Alexa ranking only goes down by having more people visiting your site who have the Alexa toolbar installed.

Are you seeing a big decrease in number of visitors though? It sounds like you're still having a lot of visitors, so maybe your main traffic source is not even search engine.

And Alexa is not an accurate indicator of your website traffic. I'm sure you already know that.

Google seems to prefer brand new websites for a short period of time, before it gets them back down to planet Earth. How old is your domain exactly? It could make all the difference.

You might want to check out the following link to see how Page Rank is determined once a domain settles down and receives normal treatment from Google
How is Google Page Rank determined? | Website Design, Search Engine Optimization and Social Media Marketing by Backbone Media, Inc.

Nobody knows the Google algorithms. Maybe you have too many pop-ups/pop-unders on your page? Try to remove them and see how your page rank.

I think your ranking went down due to last few panda updates.
Alexa ranking depends on many factors like backlinkinks, traffic from search engines etc

Stat circulating active content on your site like related photos and videos.

I offer a back linking service in my SIG for only $5. My service is completely manual and I make quality comments on HIGH PR (dofollow) blogs linking back to your site or URL. I Allyson over deliver. This or other services like that can really help.

You can do the work yourself and start building good back links for free if you wanted.

Does it really matter?

You're getting a 1000 visitors per day and I'd rather have that than high PR any day of the week.

James Scholes

One of the most important factor is relevance. Google calculates the relevance according to the activities made by visitors- how long they stay in your homepage, do they click to your inner pages etc. You can check your relevance by viewing the Bounce Rate factor in Google analytics. Try to keep it under 50%.

I think you mistake "page rank" with google position/rank (SERP).
SERP = Search Engine Ranking Position

If you lose page rank, you lose it because backlinks can come and go and they can vanish. You might have had backlinks but now the sites with the links pushed them down into oblivion, therefore you lose page rank.

Google Position: SIMILAR , but bouncing in the Google position is normal, for example other people can simply overtake you, get more links, you sink down...constant link building will also cause your position to fluctuate.

Hardly ever things are "static" in Google..site come, sites go, backlinks go, competitors come...you have to keep building links, supply fresh content to the site etc.